Why doesn't App Store use my gift card balance for purchases
Why doesn't App Store use my gift card balance for purchases
Why doesn't App Store use my gift card balance for purchases
You aren't a family organiser in family sharing, nor are gifting content ? Neither of those will use your account's balance.
If neither of those then your balance should automatically be, if not : https://www.apple.com/emea/support/itunes/contact.html
Why doesn't App Store use my gift card balance for purchases