If you're seeing an error message about in-app purchases not being allowed when trying to subscribe to premium, don't worry! There's a very simple and quick fix. You'd first need to make sure you have the Screen Time turned on.
To turn on Screen Time, go to Settings > Screen Time, then tap Turn on Screen Time. If you already use Screen Time, skip to step 3.
Tap Continue, then choose "This is My [device]" or "This is My Child's [device]."
If you're the parent or guardian of your device and want to prevent another family member from changing your settings, tap Use Screen Time Passcode to create a passcode, then re-enter the passcode to confirm. After you confirm your passcode, enter your Apple ID and password. This can be used to reset your Screen Time passcode if you forget it.
If you're setting up Screen Time on your child's device, follow the prompts until you get to Parent Passcode and enter a passcode. Re-enter the passcode to confirm. After you confirm your passcode, enter your Apple ID and password. This can be used to reset your Screen Time passcode if you forget it.
Make sure to choose a passcode that's different from the passcode you use to unlock your device. Learn what to do if you forget your Screen Time passcode.
Tap Content & Privacy Restrictions. If asked, enter your passcode, then turn on Content & Privacy Restrictions.
Tap iTunes & App Store Purchases.
Tap In-app Purchases and set to Don't Allow.
(Note: you may be required to enter your device password at some point during this process)
Why was this off to begin with?
If you have children or are sharing a device with a family member, in-app purchase permissions may have been restricted to prevent accidental purchases.