Hi, it might be to do with your previous purchases:
https://support.gaijin.net/hc/en-us/articles/115000706645-Replenishment-of-the-Gaijin-Store-Balance
"Replenishment of Gaijin balance with a bank card, PayPal, Amazon Pay, Google Pay, AliPay, WeChat or Steam is possible only on a condition that account was used to make purchases in our store https://store.gaijin.net/ during previous 12 months. In case o... Read more
https://support.gaijin.net/hc/en-us/articles/115000706645-Replenishment-of-the-Gaijin-Store-Balance
"Replenishment of Gaijin balance with a bank card, PayPal, Amazon Pay, Google Pay, AliPay, WeChat or Steam is possible only on a condition that account was used to make purchases in our store https://store.gaijin.net/ during previous 12 months. In case o... Read more