Do you mean NFC payments in Europe or Mi Pay ? because Mi Pay is only available in China I think.
If you're talking about NFC Payments in Europe you have to go to Settings > More > Secure Element position > select HCE wallet,
then your card have to be set up in an app like Android Pay if it's available in your country or in your specific bank's app if your country is using Paylib, I don't know if there is other methods of payments in Europe.
Hope it helped you, if you have set up your credit card in the correct app and it's still not working, verify in Settings > More > Tap & Pay > Payment default > choose the correct app (this one you set up with the credit card).