6 months ago - magazine2 - Direct link
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 of Steam replenishment, only purchases made at least one month before the replenishment date are counted."

Please read the article to see if it may be the cause of your issue.