Can’t update pages and plugins not responding

When trying to update a page on a clients WordPress, I receive this message: Updating failed. Error message: Could not update post in the database.
Upon research, I’ve tried to disable the Gutenberg plugin and activate the Classic editor, only this is not working.
I can install the Classic editor, but when pressing ‘activate’ I get the message that it’s been activated, however, it hasn’t.
This is the same for when trying to deactivate Gutenberg, the message says it’s deactivated however it is not. Any ideas? Thanks