If you use WordPress’ built-in theme editor, WordPress will try to access your site with the changes before saving, so you don’t accidentally end up saving corrupted code and locking yourself out of your website.
However, this does not work on our hosting, due to this security system:
As far as I know, WordPress does not provide a way to bypass these checks when using the theme editor. So if you want to edit the theme files, you’ll need to do it through FTP or the file manager.