404 page does not display

My 404 page is not displaying. When a visitor tries to access a URL that doesn’t exist on my domain, it returns:

Not Found
The requested URL /404 was not found on this server.
Additionally, a 404 Not Found error was encountered while trying to use an ErrorDocument to handle the request.

examples:
https://aiisf.org/?page_id=4 (regular page that displays correctly)
https://aiisf.org/about (lack of styled 404 page)

this is on a white page, plain message, no styling, no theme.
I have conformed that I have 404.php residing in the correct theme folder. Is there a setting that I need to activate in order for the 404 page to display?

thanks