The WordPress file editor is disabled for our Managed WordPress customers by default.
The WordPress file editor is considered a security risk and we do not recommend that you use it. Furthermore, if you accidentally save a PHP file with errors it can completely break your site and block you from accessing the WordPress Admin.
We recommend that you download files via FTP, edit and upload, or use a code editor with built-in FTP support.
Alternatively, the Plesk Panel includes a code editor as well, and it does not have the inherent risks of the WordPress file editor.