Can I use the credit on my phone to buy gift cards

Can I use the credit on my phone to buy gift card

If you mean your Apple Account balance, see -> What you can buy with your Apple Gift Card or Apple Account balance -> What you can buy with your Apple Gift Card or Apple Account balance – Apple Support (UK)

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• You can't use your Apple Account balance to send gifts or gift cards.
• If you use Family Sharing, your family members can't spend your Apple Account balance. Funds that you add to your Apple Account balance are only available to you. Find out more about how purchases are billed to your Apple Account balance.
• Some purchases might require you to have a credit card on file, even if you use your Apple Account balance to make the purchase.

You need to add another payment method to your Apple ID (a credit card or debit card) , see the following Apple articles:

Add a payment method to your Apple ID

Add a payment method to your Apple ID – Apple Support (UK)

Payment methods you can use with your Apple ID

Payment methods you can use with your Apple ID – Apple Support (UK)

fyi: Not all credit & debit cards seem to work with App Store / iTunes Store, try a different one, or preferably a credit card. If your card is not accepted as a paymenf method, you need to contact your bank.

If you update your payment method or add funds to your Apple Account balance, It would take about 1 to 3 business days in order to process.

If you're still unable to update your payment method, Contact Apple Support.

If you still need help, get more help and assistant from Apple directly, see Contact Apple for support and service -> Contact Apple for support and service – Apple Support (UK) 

For a succinct reply to your question, no.

From: How apps, content, and subscriptions from Apple are billed - Apple Support

- You can’t use your Apple Account balance to buy gifts or gift cards.