Not being able to purchase coins could be related to many reasons.

It is better to start by checking if there are any restrictions on the device. You can check your restriction options by going to Settings > Screen Time > Contact & Privacy Restrictions > iTunes & App Store Purchases > In-app Purchases. If you want to make a purchase, you should allow In-app Purchases on this screen.

For more information, you can check out the following link: https://support.apple.com/en-us/102470

Your internet connection should be stable in order to make a purchase. We suggest that you use Wi-Fi or at least a 3G/4G/LTE connection while purchasing any package.

There might be temporary problems with the store where the game was downloaded from. All in-app purchases are made through the App Store and there is a chance that the store is not able to respond to in-app purchases. There might be temporary problems with the store. You can also restart your device and try again.

Your payment information might be incorrect that’s why you are not able to complete in-app purchases.

There is a chance that the payment method you are using is not accepted by the store. To see available payment methods in stores: https://support.apple.com/en-us/111741

If you are still unable to make a purchase, you can contact us and you can also contact Apple Customer Support. To contact Apple Customer Support, you can check out the following link: https://getsupport.apple.com/