Not being able to purchase coins could be related to many reasons.
Your internet connection should be stable in order to make purchases. It is suggested that you use Wi-Fi or at least a 3G/4G/LTE/5G connection while purchasing coins.
There might be temporary problems with the affiliated store where the game was downloaded from. All in-app purchases are made through the store, and there is a chance that the store cannot respond to in-app purchases.
Your payment information might need to be corrected, hindering you from completing in-app purchases. The store may not be accepting the payment method you are using. To see available payment methods in store: Available payment methods in Google Play Store
If you are still unable to make a purchase, please get in touch with us.
To get in touch with us, you can click on the "Contact Us" button on the upper right corner of the screen, and we will be happy to assist you further.