Could not install [APP]. The requested app is not available or doesn't exist.

I've created a new app on App Store Connect. Setup everything like all the forms, the build, users, internal testing group, and then sent the invitation emails for testing. Now users can see the app in the TestFlight but when they try to install it, they get the error below.

Could not install [APP]. The requested app is not available or doesn't exist.

I've sent an email to Apple support couple of days ago but still no response.

Saw some comments on the internet that you need to send your app for review and once it's approved you can use TestFlight. So I sent the app for review with a long description about how I'm not able to use TestFlight but still on wait.

Tried to create a public TestFlight link but I get this error: "Beta contract is missing for the app." but there's actually no contract anywhere.

What do I do? This is completely unacceptable. Why is the TestFlight not working? Why are the error messages not helpful?