I just want to clarify on what you said:
Do you mean the Page post type is enabled in X > Settings > Permissions > General for the user level like Adminstrator? It could be that you’re using an Editor account and the Page post type is disabled for Editor user level.
If the page post type is enabled for all user level, a third-party plugin could be causing this because Cornerstone couldn’t be disabled apart from the permission settings. Please kindly test for a plugin conflict and let us know the result. You can do this by deactivating all third party plugins, and seeing if the problem remains. If it’s fixed, you’ll know a plugin caused the problem, and you can narrow down which one by reactivating them one at a time.
Only if you’ve verified the user permission and performed a third-party plugin conflict test and they don’t help, please give us WordPress Admin and cPanel access in a Secure Note.