Structured Data or Schema Markup is the most useful and important feature to promote, index, add rich snippet contents of your website or blog properly in Search Engine result like Google, Bing etc.
Learn how to fix structured data errors in Blogger on this tutorial.
First backup your theme and next go to your Blogger Dashboard -> Theme -> Edit HTML.
Package 1: Search for <b:includable id='post' var='post'> because we have to replace the following code.
<div class='post hentry uncustomized-post-template' itemprop='blogPost' itemscope='itemscope' itemtype='http://schema.org/BlogPosting'>
<b:if cond='data:post.firstImageUrl'>
<meta expr:content='data:post.firstImageUrl' itemprop='image_url'/>
</b:if>
<meta expr:content='data:blog.blogId' itemprop='blogId'/>
<meta expr:content='data:post.id' itemprop='postId'/>
Select the code like the image snippet.
Selected to replace code 1
Replace with the following code.
<div class='post h-entry uncustomized-post-template' itemscope='itemscope' itemtype='http://schema.org/BlogPosting'>
<!-- Twitter Card Tags --> 
<meta content='@compromath' name='twitter:site'/>
<meta content='@Elliyas_Ahmed' name='twitter:creator'/>
<b:if cond='data:post.firstImageUrl'>
   <meta content='summary_large_image' name='twitter:card'/>
   <!-- summary_large_image or any other your card types --> 
   <meta expr:content='data:post.firstImageUrl' name='twitter:image'/>
   <b:else/>
   <meta content='summary' name='twitter:card'/>
   <b:if cond='data:blog.postImageThumbnailUrl'>
      <meta expr:content='data:blog.postImageThumbnailUrl' name='twitter:image'/>
   </b:if>
</b:if>
<b:if cond='data:blog.pageName'>
   <meta expr:content='data:blog.pageName' name='twitter:title'/>
   <b:else/>
   <meta expr:content='data:blog.title' name='twitter:title'/>
</b:if>
<b:if cond='data:blog.metaDescription'>
   <meta expr:content='data:blog.metaDescription' name='twitter:description'/>
   <b:else/>
   <meta expr:content='data:post.snippet' name='twitter:description'/>
</b:if>
<meta expr:content='data:post.sharePostUrl' name='twitter:url'/>
<!-- /Twitter Card Tags --> <!-- Pinterest Rich Pins Tags --> 
<meta expr:content='data:post.timestampISO8601' property='article:published_time'/>
<meta content='Technology' property='article:section'/>
<b:loop values='data:post.labels' var='label'>
   <meta expr:content='data:label.name' property='article:tag'/>
</b:loop>
<!-- /Pinterest Rich Pins Tags --> 
<meta expr:itemid='data:post.link ? data:post.link : data:post.url' itemType='https://schema.org/WebPage' itemprop='mainEntityOfPage' itemscope='itemscope'/>
<meta expr:content='data:post.timestamp' itemprop='datePublished'/>
<meta expr:content='data:post.lastUpdatedISO8601' itemprop='dateModified'/>
<meta expr:content='data:post.author' itemprop='author'/>
<div itemprop='publisher' itemscope='itemscope' itemtype='https://schema.org/Organization'>
   <div itemprop='logo' itemscope='itemscope' itemtype='https://schema.org/ImageObject'>
      <image alt='compromath' itemprop='url' src='http://1.bp.blogspot.com/-FIEZPMAnW7Y/WHMUbTqioFI/AAAAAAAABC0/Bp3k9fcxMmM-raXqsc--hOFZcm2DrROwgCK4B/s1600/logo%2Bcompromath.png' style='display:none'/>
      <meta alt='compromath' content='http://1.bp.blogspot.com/-FIEZPMAnW7Y/WHMUbTqioFI/AAAAAAAABC0/Bp3k9fcxMmM-raXqsc--hOFZcm2DrROwgCK4B/s1600/logo%2Bcompromath.png' itemprop='image' style='display:none'/>
      <meta content='600' itemprop='width'/>
      <meta content='600' itemprop='height'/>
   </div>
   <meta expr:content='data:blog.title' itemprop='name'/>
</div>
<div itemprop='image' itemscope='itemscope' itemtype='https://schema.org/ImageObject'>
   <image alt='compromath' itemprop='url' src='http://1.bp.blogspot.com/-FIEZPMAnW7Y/WHMUbTqioFI/AAAAAAAABC0/Bp3k9fcxMmM-raXqsc--hOFZcm2DrROwgCK4B/s1600/logo%2Bcompromath.png' style='display:none'/>
   <meta alt='compromath' content='http://1.bp.blogspot.com/-FIEZPMAnW7Y/WHMUbTqioFI/AAAAAAAABC0/Bp3k9fcxMmM-raXqsc--hOFZcm2DrROwgCK4B/s1600/logo%2Bcompromath.png' itemprop='image' style='display:none'/>
   <meta content='800' itemprop='width'/>
   <!--Images should be at least 696 pixels wide.--> 
   <meta content='800' itemprop='height'/>
</div>
<meta expr:content='data:blog.blogId'/>
<meta expr:content='data:post.id'/>
N.B. You have to customize as your Twitter username and replace http://1.bp.blogspot.com/-FIEZPMAnW7Y/WHMUbTqioFI/AAAAAAAABC0/Bp3k9fcxMmM-raXqsc--hOFZcm2DrROwgCK4B/s1600/logo%2Bcompromath.png with the Logo URL of your Blog/Website.
Package 2: Search for <b:includable id='mobile-post' var='post'> because we have to replace the following code.
<div class='post hentry uncustomized-post-template' itemscope='itemscope' itemtype='http://schema.org/BlogPosting'>
<b:if cond='data:post.thumbnailUrl'>
<meta expr:content='data:post.thumbnailUrl' itemprop='image_url'/>
</b:if>
<meta expr:content='data:blog.blogId' itemprop='blogId'/>
<meta expr:content='data:post.id' itemprop='postId'/>
Select the code like the image snippet.
Selected to replace code 2
Replace with the following code.
<div class='post h-entry uncustomized-post-template' itemprop='blogPost' itemscope='itemscope' itemtype='http://schema.org/BlogPosting'>
<meta expr:itemid='data:post.link ? data:post.link : data:post.url' itemType='https://schema.org/WebPage' itemprop='mainEntityOfPage' itemscope='itemscope'/>
<meta expr:content='data:post.timestamp' itemprop='datePublished'/>
<meta expr:content='data:post.lastUpdatedISO8601' itemprop='dateModified'/>
<meta expr:content='data:post.author' name='author'/>
<div itemprop='publisher' itemscope='itemscope' itemtype='https://schema.org/Organization'>
   <div itemprop='logo' itemscope='itemscope' itemtype='https://schema.org/ImageObject'>
      <image alt='compromath' itemprop='url' src='http://1.bp.blogspot.com/-FIEZPMAnW7Y/WHMUbTqioFI/AAAAAAAABC0/Bp3k9fcxMmM-raXqsc--hOFZcm2DrROwgCK4B/s1600/logo%2Bcompromath.png' style='display:none'/>
      <meta alt='compromath' content='http://1.bp.blogspot.com/-FIEZPMAnW7Y/WHMUbTqioFI/AAAAAAAABC0/Bp3k9fcxMmM-raXqsc--hOFZcm2DrROwgCK4B/s1600/logo%2Bcompromath.png' itemprop='image' style='display:none'/>
      <meta content='600' itemprop='width'/>
      <meta content='400' itemprop='height'/>
   </div>
   <meta expr:content='data:blog.title' itemprop='name'/>
</div>
<b:if cond='data:post.thumbnailUrl'>
   <meta expr:content='data:post.thumbnailUrl' itemprop='url'/>
</b:if>
<meta expr:content='data:blog.blogId'/>
<meta expr:content='data:post.id'/>
N.B. Replace http://1.bp.blogspot.com/-FIEZPMAnW7Y/WHMUbTqioFI/AAAAAAAABC0/Bp3k9fcxMmM-raXqsc--hOFZcm2DrROwgCK4B/s1600/logo%2Bcompromath.png with the Logo URL of your Blog/Website.
Package 3: Search For 'post-title entry-title' itemprop='name' and replace with 'post-title entry-title' itemprop='headline'

#blogID Structured Error

Search for <meta expr:content='data:blog.blogId' itemprop='blogId'/> and replace with <meta expr:content='data:blog.blogId'/>
You can get this twice after hitting enter. You have to change both times.

#postID Structured Error

Search for <meta expr:content='data:post.id' itemprop='postId'/> and replace with <meta expr:content='data:post.id'/>
You can get this twice after hitting enter. You have to change both times.

#headline Structured Error

Search for 'post-title entry-title' itemprop='name'
and replace with 'post-title entry-title' itemprop='headline'
You can get this twice after hitting enter. You have to change both times.

#image and image_url Structured Error

Search for <b:if cond='data:post.firstImageUrl'> and replace the following code:
<b:if cond='data:post.firstImageUrl'>
  <meta expr:content='data:post.firstImageUrl' itemprop='image_url'/>
</b:if>
with
<!-- Commented code
<b:if cond='data:post.firstImageUrl'>
<meta expr:content='data:post.firstImageUrl' itemprop='image_url'/>
</b:if>
End Commented code -->
<div itemprop='image' itemscope='itemscope' itemtype='https://schema.org/ImageObject'>
<b:if cond='data:post.firstImageUrl'>
<img style='display:none;' expr:src='data:post.firstImageUrl' alt='Textual description of firstImageUrl'/>
<meta itemprop='image' expr:content='data:post.firstImageUrl'/>
</b:if>
<meta itemprop='width' content='800'/> <!--Images should be at least 696 pixels wide.-->
<meta itemprop='height' content='800'/>
</div>

#mainEntityOfPage Structured Error

To fix mainEntityOfPage error you have to find the following code. To find search for <b:includable id='post' var='post'>
So your replaceable code:
<b:includable id='post' var='post'>
<div class='post hentry uncustomized-post-template' itemprop='blogPost' itemscope='itemscope' itemtype='http://schema.org/BlogPosting'>
Replaced with:
<b:includable id='post' var='post'>
<div class='post hentry uncustomized-post-template' itemscope='itemscope' itemtype='http://schema.org/BlogPosting'>
<meta itemscope='itemscope' itemprop='mainEntityOfPage'  itemType='https://schema.org/WebPage' expr:itemid='data:post.link' content= 'data:post.link : data:post.url'/>
Again search for <b:includable id='mobile-post' var='post'> and your replaceable code is:
<b:includable id='mobile-post' var='post'>
<div class='post hentry uncustomized-post-template' itemscope='itemscope' itemtype='http://schema.org/BlogPosting'>
Replaced with:
<b:includable id='mobile-post' var='post'>
<!-- Commented code
<div class='post hentry uncustomized-post-template' itemscope='itemscope' itemtype='http://schema.org/BlogPosting'>
End Commented code -->
<div class='post hentry uncustomized-post-template' itemscope='itemscope' itemtype='http://schema.org/BlogPosting'>
<meta expr:itemid='data:post.link' content= 'data:post.link : data:post.url' itemType='https://schema.org/WebPage' itemprop='mainEntityOfPage' itemscope='itemscope'/>

#dateModified and datePublished Structured Error

Search for uncustomized-post-template and paste the following just after the divwith the above attribute
<meta expr:itemid='data:post.link ? data:post.link : data:post.url' itemType='https://schema.org/WebPage' itemprop='mainEntityOfPage' itemscope='itemscope'/>
<meta expr:content='data:post.timestamp' itemprop='datePublished'/>
<meta expr:content='data:post.lastUpdatedISO8601' itemprop='dateModified'/>

#Publisher Structured Error

To fix publisher error search for uncustomized-post-template and paste the following code just after the div included with the above attribute.
<div itemprop='publisher' itemscope='' itemtype='https://schema.org/Organization'>
<div itemprop='logo' itemscope='' itemtype='https://schema.org/ImageObject' style='display:none;'>
<img src='PUT LOGO URL HERE'/>
<meta content='PUT LOGO URL HERE' itemprop='url'/>
<meta content='600' itemprop='width'/>
<meta content='400' itemprop='height'/>
</div>
<meta expr:content='data:blog.title' itemprop='name'/>
</div>