We signed in wirh itunes gift card, why do you want an alternate pay method

Hi Bassetmamma,

welcome to the Apple (user-to-user) Community.

Re: "We signed in wirh itunes gift card,

why do you want an alternate pay method"

We are all Apple users, like yourself, in this Community, so we have no access to your accounts or details to help check the purchase out for you. We'd like to help, however:

See the article below for what payment methods you can use in your country or region:

Payment methods that you can use with your Apple ID – Apple Support

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Re: "Why does the app require another payment method when I signed in with an iTunes gift card?"

Apple Support has articles online to help with using Gift Cards:

Seems, if gift cards are accepted as a payment method in your country, you may need to "redeem" the gift card, and use the balance for purchases from Apple:

How to redeem your Apple Gift Card or App Store & iTunes gift card - Apple Support

NB: article also says:

"Some purchases might require that you have a credit card on file, even if you use your Apple Account balance to make the purchase".

Seems for some purchases, Apple Account balance can't be used:

What you can buy with your Apple Gift Card or Apple Account balance – Apple Support 

How to: Check your gift card balance

If needed:

App Store Support (website with info. and contact option)

App Store - Official Apple Support

All the best :-)