I'm European and I'm yet to find a credit card field where I can't put my debit card number. I'm not sure I understand what you're saying.
Debit card number is not your IBAN, right? Because that's the only number on my bank card and it never works (because all forms want your "credit card" number, I thought).
Oh, no, it's not your IBAN. It's a number that looks like a credit card number, but the card it points to is actually a debit card.
What kind of credit card form doesn't allow debit cards? I've never seen one and been shopping all over Europe, both online and in person.
Oh, you mean you mostly use non-VISA/MC debit cards like we do in Canada? (we have Interac)
how do you pay for things online?
In Belgium something called MrCash is popular, The Netherlands has iDeal supported by pretty much all banks. Other than that: bank transfers, direct debit, or third-party things like PayPal (or in my case, Bitcoin).