Missing Beta Entitlement - Where can I find this?

I am attempting to test a file in TestFlight. I managed to upload the ipa file however, when I try to add the file to TestFlight for an internal tester the option to add the app is not available. When I check the External Testing Status in the Activities tab says "Missing Beta Entitlement." Where do I find information about this? There doesn't seem to be much information about this issue.