i have money added to my google play account, so i can buy gems in Titan revenge

I wish to Order an amount of gems per google pay, and the paymend website force me to use a Creditcard.

Honestly i have one prepaid credit card, but heres no money.

Why do the game force me to pay with credit card, if i have money on my google play account? these is the only game that happens. on other games it takes the google pay money first.