WordPress 404 error means that the server was unable to find the requested page. Why? Either the page no longer exists or the URL has changed. And for this article, I will assume that you already checked your trash and made sure you haven’t deleted the requested page by mistake.
How to Fix WordPress Posts Returning 404 Error
In most of the cases, the WordPress permalink structure is to be blamed for causing 404 errors, and it usually happens after WordPress website migration to another domain or WordPress transfer to another hosting.
The easiest way to solve this issue if it was caused by the permalink structure is just to save the permalink structure once again.
WARNING: Changing your permalinks settings can seriously impact your search engine visibility. It should almost never be done on a live website.