The control panel tends to be a bit picky with URLs. There are some security filters that have the tendency to just block the request if they see anything they don’t like.
If this happens, there isn’t a lot you can do about it in the control panel. Fortunately, what the Error Pages tool does is not rocket science, and is pretty easy to do yourself.
Simply go into the file manager, navigate to the folder of your website, and file the file with the name .htaccess. If it doesn’t exist, you can create one with that name.
Within that file simply add a line like this:
ErrorDocument 404 http://chris-hatfield-copywriter.free.nf/pages/404.html
Then, simply save the file and you’ll have set the error page!