Why I can’t use the money on my Apple ID to buy something on my game

It would be a lot easier to provide you with a reason if you told us what makes you think this. Quote exact error messages or a description of what you are attempting to do and how it fails to do that. Apple has a broad statement in: What you can buy with your Apple Gift Card or Apple Account balance - Apple Support

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

Some purchases aren’t charged to your Apple Account balance

- If you're in a Family Sharing group, purchases that you make are charged to your personal Apple Account balance. If you don't have enough Apple Account balance to pay for the purchase, the remainder is charged to the family organizer if purchase sharing is turned on.

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

- Some subscriptions might not be charged to your Apple Account balance.

Note in terms of, "you need to set your Apple Account balance as your default payment" you do not need to do this. It is automatically used first (or not in some cases) as outlined in How apps, content, and subscriptions from Apple are billed - Apple Support You cannot change the position of it as a default payment method.

See -> How to redeem your Apple Gift Card or App Store & iTunes gift card

How to redeem your Apple Gift Card or App Store & iTunes gift card – Apple Support (UK)

When redeeming a gift card, the amount will be credited to your (1) Apple Account balance and (2) you need to set your Apple Account balance as your default payment.

(1)

See -> Check your Apple Account balance

Check your Apple Account balance – Apple Support (UK)

(2)

See -> Add a payment method to your Apple ID

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

If you already use your Apple Account balance, the Apple Gift Card or App Store & iTunes gift card is automatically added to your Apple Account balance.

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.

See also -> 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.

If you have a valid payment method on file, instead of buying or order an App Store & iTunes gift card, you can Add money to your Apple Account balance anytime, no gift card needed, see -> Apple Gift Card - Apple (UK)

If you still need help, get support from Apple directly -> Get Support or call Apple in your region see -> Contact Apple for support and service or check -> here for more support and assistance.