And if I have no credit card?

(I have no credit card. I approach 6 decades in age.)

Don't debit cards work as well?

If you have no debit or credit card, why would any for-profit enterprise want to interact with you in the first place?

Debit cards work if they're Visa or MasterCard, probably. Many countries have national card schemes for debit cards that do not work.

I doubt it has anything to do with verifying age though. More likely they know people are hesitant to sign up and pay for youtube, but if they can get your credit card in other ways, then later it easier to get you to pay for something because then you just have to press "ok" since your details are already entered.

Plus this gives them your identity, which is relevant because their business model nowadays is based around tracking people.

eIDAS + smart card reader. :)

But I guess, google still wants to make the check passable.