I have tried to modify the 404 page settings in the theme setting but nothing has changed in the frontend page. Even the default content in the settings is different from what I see in the frontend.(The text is different)
It looks like that the error isn’t related to 404 page as the page may be found, but there is a content related issue. It is hard to comment further as we don’t know the exact website setup.
In this case the text comes from content-none.php file.