I have iTunes credit, and when I try to buy from the Top War game, I have a problem with payment

I have iTunes credit, 

See -> Change, add or remove Apple ID payment methods

Change, add or remove Apple ID payment methods – Apple Support (UK)

• set your Apple ID balance as your default payment.

If you update your payment method or add funds to your Apple ID balance, It would take about 1 to 3 business days in order to process.

If you're still unable to update your payment method, Contact Apple Support.

Click here for more support and assistance.

For your 2nd screenshot, see Require a password for App Store and iTunes purchases

Require a password for App Store and iTunes purchases - Apple Support