I cant buy uc with apple credit

In-app purchases can be done with the payment method that is linked to your Apple ID account.

Payment methods that you can use with your Apple ID (by country) - Apple Support

In many countries, that includes account balance as a possibility.

Add money to your Apple Account balance - Apple Support

How to redeem your App Store & iTunes gift card - Apple Support

Check your Apple Account balance - Apple Support

apple credit

If this credit is different from the aforementioned account balance, then it won’t work for in-app purchases.

You cannot purchase pubg uc with your Apple Account balance.

Apple (App Store) doesn't sell pubg uc.

Search in your country how to pay pubg uc.