How to ensure gutenberg/block editor works for custom post types?

I’m troubleshooting a client’s website which has its own custom theme and one of the issues is that the website owner cannot edit and create new posts because their WordPress can only edit/make posts via the classic/text editor.

In my Gutenberg settings I am unable to activate ACF because the panels section in preferences is not visible.

WordPress 6.5.3
ACF 6.2.9
Gutenberg 18.4
PHP 8.0.3

I’ve disabled the classic editor plugin and managed to enable the Gutenberg editor on Posts, but for some reason the additional custom fields are refusing to switch over to Gutenberg and remain on the classic html text editor.

I’ve disabled all plugins and activated a generic theme to test if the issue still presents, and now posts with custom fields no longer appear in the sidebar at all and the Gutenberg settings issue also persists.

I am starting to think that this comes down to a js issue, but I have no idea how to begin fixing this. Can anyone help?