When you are unable to purchase a subscription, please check if In-App Purchases within your phone settings is turned on. To check and turn on In-App Purchases, please follow the steps below:
  1. Go to device Settings
  2. Tap Screen Time
  3. Tap Content & Privacy Restrictions
  4. Turn on Content & Privacy Restrictions
  5. Tap iTunes & App Store Purchases 
  6. Tap In-app Purchases
  7. Check Allow