If you attempted to make a payment in-game with real life currency and it didn't get through, make sure that you have Steam Overlay enabled in the Steam Settings for GWENT. To do so:
1. Go to Settings in Steam -> In-Game -> Enable the Steam Overlay while in-game
2. In order to enable Overlay for GWENT, right click on GWENT in Steam library -> Properties -> General -> mark "Enable the Steam overlay while in-game"
If the issue persists, please, try changing the currency in your GOG.COM account settings to your current place of residence, and see if it helps: https://www.gog.com/account/settings/personal
If the above solution doesn't help, please refer to this article.