When I enter my card info and press "Upgrade" button some kind of error field appears, but there is no text so I don't know what is wrong.

Please logout, then login again and let me know if it works.

Logout:
https://www.mixo.dj/logout-website

Login:
https://www.mixo.dj/gold

Did you have any luck with this? I had a look at the logs and can see your card was declined for “generic reasons” by your bank.

Please make sure your details are correct, try a different card or try Apple/Google pay on mobile.