In a lot of countries you cannot just use your debit card directly on Steam like you would a credit card which includes just entering the numbers and you are good to go. Or in some cases it directs you to the credit card provider where you have to authenticate the payment.
Here in NL we have to use iDeal if we want to pay online with most banks, this requires us to 2FA the payment by scanning a code using their app. iDeal will become Wero and will become an international system since a lot of countries had something similar to the Dutch iDeal.
Make sure 2FA is enables on all your banking