It depends on the country, not all debit cards can be used for online purchases or outside the system of the bank to which they belong, that is, they do not have CCV for example.
This may seem strange if you live in the US, but it is quite common in other countries.
Viewing post in Paying with Debit Card?
But Steam has agreements in many countries and allows the use of cards from those countries, they are considered local purchases. Can this be the case?
I live in Chile and for example, you can use a local debit card with steam, but the same debit card is not served in itch, because in itch it appears as an international purchase.
I make my comments based on what I have seen in Chile, it could be totally different in your country.