There may be several reasons why you cannot make in-app purchases.

If you don't see a price or there's a wrong one, it might be because your game cannot connect to the store.

In that case:

• check that your device has a stable internet connection
• make sure you have a valid payment method on the App Store.

If the solutions above didn’t help, you could look for them here.

Some limitations your parents/guardians set for you may affect the game purchases. Check more here: Apple Support - Family Sharing.