I can’t add an external tester in TestFlight because the Add button is greyed out

The pragmatic way to ensure all testers are off the app is to wait the 30 days for the build to expire and have them manually grab the store version.

You could also upload a new beta build instructing them to do the same. That way they get an email from Apple with the news you want to convey to them and a limited duration to get them all off that build.

I was having this same problem. After contacting Apple Support it turned out the problem was I did not have the Paid Apps contract in effect on my account. To put the contract into effect:

1. Log onto iTunes Connect with an account with Admin or Legal roles.
2. On the dashboard select Agreements, Tax and Banking.
3. Click request button next to the Paid Applications Contract.