404 page not showing up

I’ve followed the instructions in the Codex for making sure there is a 404.php page in the child theme directory, the .htaccess file has been updated with the line of text per the codex, but when I look for a non-existant page, all I get is a white screen, no 404 or anything else.

It is possible I did the php code wrong as I don’t know php, just copy and paste it. Can someone help?

Site is https://sewcozyinteriors.net