I would send an email to support@airtable.com, and report back here with what they tell you!

I would also try a good ol’ reset. Either refresh your Airtable window/tab, go back out to the home screen and back into your base, or log out and back in entirely.

Also make sure that you’re attempting to add an app to a base that is within the Workspace you upgraded to Pro. “Pro” features are granted per workspace, its possible to have another workspace still on the “free” tier.