If you receive an error when subscribing, please:

1. be sure to have not activated any RESTRICTION on the System settings/Screen Time.
2. try to Logout-Login from iCloud
3. check your payment method, in the iCloud user of your System Settings
4. restart the device

- https://www.digitaltrends.com/mobile/how-to-turn-off-in-app-purchases-ios-iphone-ipad/

- https://support.apple.com/en-us/HT204396