My purchase could not be completed … how I can fix it ? And I can’t add another method cuz I live in iran and only way for purchase is American iTunes card

Hi ghazal152,

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

You might want to check Apple Support's online article (link below):

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

Eg: 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.

Some stores and apps might not be available in your country or region.

Find out what's available in your country or region."

All the best :-)