This means one of you used VPN and his store currency was changed. Which in turns means inability to purchase anything, not just gifts (if your actual country of residence doesn't match with currency). You have to wait until support checks your accounts.
Please note that it's forbidden to share your communication with support publicly as it may contains sensitive personal data.
Also, did you try today and if it's still not working, provide the nicknames of your friends?
Again, please tell the nickname of your friend you're trying to send the gift to.
Because the pricing in different currencies may be lower than actual USD/EUR conversion rate. It is done to prevent gift system abuse, but all of European countries (except UK) retained pricing in EUR like it was before. If you are encountering such issue (both users are either in US or Europe but can't gift to each other), please provide nicknames.
Again, please provide the nickname who you trying to gift to, just your account alone is useless.
So it seems that issue isn't the currency, but your friend changing his nickname. Try sending the gift using the new one.
He changed his nick to https://warthunder.com/en/community/userinfo/?nick=bunnling
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