The AppCenter is different and I don't think the settings there really matter to simply enable a Page Tab.
In your Facebook Page area, I see the drop-down with a link to your site. When I click it, I get a blank white area. The cause of this is due to a server restriction on your site. If you go to the browser console while on your Facebook Page Tab, you'll see the following error message:
Refused to display 'https://YOURSITE.com/' in a frame because it set 'X-Frame-Options' to 'SAMEORIGIN, SAMEORIGIN'.
You'll need to remove that X-Frame-Options header from the response. That could be in your Apache configuration, a firewall, or a security extension within Joomla. That's not something we can fix on our end, but I hope that helps you narrow things down.
Thanks,
Alex