Why can't I pay with Netflix Gift Card?

I entered the pin, but there's always this error message as you can see in the picture. Does anyone know why this is?

Just try again later. If it does not work then contact the support.

Try writing it down with letters

You should basically first deposit a valid payment method such as a credit card, only then go gift vouchers. So you ensure against it, that minors enter into contracts that they may not have. Is the same game on Amazon.