Try these troubleshooting steps:

1. Go to the Settings of your device => General => Restrictions => check if the In-App purchases are ON.

2. Confirm your Apple ID in your device's Settings menu:

• Close the game.
  
• Log out from the AppStore (device Settings => iTunes & AppStore).
  
• Launch the game and attempt to purchase Gems.
  
• Enter your Apple ID profile data in the window that appears.
  
• Confirm your purchase.
  

3. Try to use different Apple ID to proceed a payment.

If the issue persists, feel free to submit a ticket to our Support Team.