You may have to ask for approval of your purchases if you’re under 18 and your device is associated with Family Sharing.

If you see a system window telling you to Ask to Buy, you won't be able to make the purchase by yourself. In that case, send a purchase request to your Family Sharing organizer or decide to go without making it. If the organizer confirms the purchase you want to make using the card associated with Family Sharing, it will be downloaded to your device automatically.

If your device is marked as Parent/Guardian and your family members can't make purchases through Family Sharing, follow these steps.
1. Go to the device settings.
2. click iCloud -> Family. Select the family member who can't make the purchase.
3. Switch the Ask to Buy feature off and then back on, and retry making the purchase on the device from which the request was sent.