You cannot use an existing Apple Account balance to buy or send new gift cards.
You can use an Apple Gift Card (the modern "all-in-one" card) to buy both physical products and digital content, including music and apps. However, if you specifically have an older, dedicated (usually a solid gray, white, silver, or gold color), you cannot directly use it to buy digital content or "iTunes" cards through standard online checkout. Key Limitations buy itunes card with apple store gift card
Depending on which card you have, you can convert its value into usable credit for apps and music: You cannot use an existing Apple Account balance
Some users have successfully exchanged or used these cards at physical Apple Store locations to facilitate a switch, though this is subject to current store policy. Important Safety Note Apple Gift Card Key Limitations Depending on which card you have,