Issue
You have paid for Heroku Connect but are only able to add the demo plan. You get an error when you attempt to add one of the paid plans: enterprise or shield.
Resolution
Check that the team associated with your app has purchased Heroku Connect. The most common cause for the error is that your team isn't licensed to use the paid edition of Heroku Connect.
Note that if you have a demo plan and your team has purchased Heroku Connect, the plan will be upgraded automatically to the enterprise plan after you have accessed the Heroku Connect dashboard for the first time. You can do this by clicking on the add-on in the Heroku Dashboard, or by using the addons:open CLI command.
$ heroku addons:open herokuconnect
If it doesn't automatically upgrade, ensure your application belongs to a Heroku Team that has the Connect entitlement.
See also: I’m getting an error when I try to provision a Heroku Connect add-on