https://www.bloggerspice.com/2015/02/how-to-hide-sidebar-in-blogger-for-full-width-post-page.html

Step #1: Log in to your Blogger Account and Go to your Blogger Dashboard

Step #2: Click on Now click on -> Template -> Edit HTML

Step #3: Now find the </b:skin> by pressing Ctrl+F (Windows) or CMD+F (Mac)

Step #4: Copy the code from below and Paste the code after/below </b:skin>
 <pre class="code">
<b:if cond='data:blog.pageType == &quot;item&quot;'>
<style>
#main-wrapper .post {
padding-left: 1px !important;
padding-right: 1px !important;
}
#main-wrapper { width:99%!important;}
.post { width:99%!important; }
#sidebar-wrapper,#sidebar-wrapper .left,#sidebar-wrapper .right, #midsidebar-wrapper .post-header-line-1, .post-footer{
display:none !important;
}
</style>
</b:if>
</pre>