At the moment if you accidentally forget to specify the .tab or .map extension for a page and visit it in order to then create, you see a message saying:
There is currently no text in this page. You can search for this page title in other pages, or search the related logs, but you do not have permission to create this page.
Though there's no edit button, being persistent enough you could specify ?action=edit manually and see another message:
You do not have permission to create this page, for the following reason:
You are not allowed to execute the action you have requested.
While doubtlessly both messages are correct, I am not allowed to create a Data namespace page without an extension explicitly specified, none of them actually writes this very reason. It is even more confusing because usually you see an error message like this when you have insufficient rights (though in that case there's a usergroup needed is mentioned.
This being explained I believe the solution is obvious: there must be the exact reason (file extension missing) specified in those messages.