There are many possibilities to cause this problem. Sometimes AppStore might not be able to tell whether you're using the paid version or the beta. A jailbroken device might also lead to the same problem.
Please try these steps and see if it helps:
1. Log out and login with your App Store account.
2. Close all the apps.
3. Cut off the connection to computers or audio interfaces.
4. Restart your iPhone/iPad.
5. Launch JamUp and try to restore purchased again.
Or you can refer to this article provided by Apple: