Pro Sites Checkout Page Not Generating

I can't seem to get my Checkout page to generate. It says in the settings that: "There was a problem finding the Checkout Page. It will be created when you save the settings on this page." But when I click Save, it still doesn't generate.

I'm currently running WP Multisite and I'm wondering if it has anything to do with Pro Sites -> Manage Sites -> Blog ID or Activation Key?

If you could please point me in the right direction, that would be awesome! :slight_smile: