Try adding something to your shopping cart and then click "Go to Checkout" (you don't have to buy it and can remove whatever you added). This often resets it.

tried it and it didn't work..

Perhaps refreshing your account?

https://www.gog.com/account/refresh

nope, it didn't work

Serbia does not use EUR. I'm rather surprised that in the past you were able to pay with EUR, since the default currency on GOG is USD (available from every country) plus any local currency if supported (e.g. EUR from countries in the Euro zone which does not include Serbia).

Maybe GOG changed the settings?

Other things I can think of:
- Did you fiddle with a VPN?
- Delete the cookies
- Logout from your account and login again
- Try from a different device
- Try the account refresh described by ConsulCaesar a few more times

If nothing helps, you could ask support.

already contacted the support. Last time the payment was in USD but the time before that it was in EURO, I just checked on my phone. Also, steam lets me pay in EURO, despite Serbia being outside the Euro zone. And my friends from Serbia can choose between USD and EURO via dropbox and I don't have such option.