Method : 1 Meta Tag Refresh ( Recommended )
Go to Blogger Dashboard › Select a blog › Switch to Template tab › Edit HTML
Now search for <head> tag (using CTRL + F )
Copy and paste below code just below/after <head> tag
<b:if cond='data:blog.pageType == "error_page"'> Oops! Page does not exist. We are redirecting you to home page. <meta content='5;URL=http://www.your-blog-url.com' http-equiv='refresh'/> </b:if>
Finally save your template › Done.
Go to Settings › Search Preferences › Custom Page Not Found ( under Error and redirections)
Click Edit and paste below code :
Click Save Changes › Done.
Now you can test the error page code is working by entering URL that doesn’t exits in your domain. (e.g. http://your-blog-url.com/404 )