Found the issue and the solution!
Basically, Apple TV logs in the sandbox user to be the default user on Apple TV automatically. That means, that a non-sandbox user gets logged and tries to make a sandbox purchase, which causes the error.
To solve this:
Settings
Users and Accounts
Sign out the Sandbox Account
Login with a normal sandbox user
Do the purchase.
Hope this helps!
Found the issue and the solution!
Basically, Apple TV logs in the sandbox user to be the default user on Apple TV automatically. That means, that a non-sandbox user gets logged and tries to make a sandbox purchase, which causes the error.
To solve this:
Settings
Users and Accounts
Sign out the Sandbox Account
Login with a normal sandbox user
Do the purchase.
Hope this helps!