iPod Did I just waste a 15€ card?

I was trying to create an iTunes account for my 10y old son who has no credit card obviously.

1. I bought a 15€ gift card as I read this would work as well.
2. I clicked on "use a code" in the iTunes Store.
3. I entered my gift card code.
4. iTunes asked me to login or create a new account I chose the latter.
5. When asked to enter credit card details or gift card, I re-entered the gift card code.
6. iTunes complained about the code no being valid.
7. When trying to restart from the beginning, the code is no longer accepted

How am I supposed to create an iTunes account for my son