how to use an itunes gift card
How do I pay for itunes ringtones by using an itunes gift card?
iPhone 6
If you haven't already done so redeem it onto your account : Redeem iTunes or Apple Music Gift Cards and content codes - Apple Support
Its value will be added to your account, and will automatically be used for your purchases, assuming that you aren't a family organiser in family sharing (organisers can't use a balance). Is that what you've tried ?