The symptoms of this error is that when a user visits a single post on their site they get a 404 page – not found error.
The user can browse all other sections of their site including the admin area. The most common cause of this issue is permalink settings in WordPress. To solve this issue a user would need to reconfigure their permalinks settings or manually update their rewrite rules.
Go to Settings » Permalinks, and simply click on Save Changes button.
This will update your permalinks settings and flush rewrite rules. In most cases this solution fixes the WordPress posts 404 error. However, if it does not work for you, then you probably need to update your .htaccess file manually.
Login to your server using FTP, and modify the .htaccess file which is located in the same location where folders like /wp-content/ and /wp-includes/ are located. The easiest thing you can do is to temporarily make the file writeable by changing the permissions to 666. Then repeat the original solution. Don’t forget to change the permissions back to 660. You can also manually add this code in your .htaccess file: