This Apple ID doesn't have permission to make In-App Purchases. You can change account permissions in App iTunes Connect.

Issue:- You are not authorized to make purchases of this InApp in Sandbox at this time.
This Apple ID doesn't have permission to make In-App Purchases. You can change account permissions in
App Store Connect.
[Environment: Sandbox]

You Need to ensure first you applied all the steps to configure in-app purchases. 

We need to verify some steps:- 

1. You create in-app purchase product ids status --- Ready to submit.
2. Make sure you added all metadata information in the in-app purchase
3. Make sure you create a external group in app store connect -> test flight like this. 
4. Your app must be approved for beta testing. 
5. your app must be available for new testers after approved beta testing. 
6. Don't miss Notify to the testers. It's always on and beta testing is available for testers.