I have $20 in my Apple ID balance from a gift card but can’t make any in app purchases

See -> Check your Apple Account balance

Check your Apple Account balance – Apple Support (UK)

or 

Tap App Store -> tap your Profile (top right of the screen) -> then you will see your Apple Account balance.

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.

I have $20 in my Apple ID balance from a gift card

If you have enough money in your Apple Account balance, all you have to do is -> you need to set your Apple Account balance as your default payment.

See -> Add a payment method to your Apple ID

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

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)

and ⤵️

• 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 followed all the steps and instructions and 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.

It still won’t work because to verify it I need the original gift card that I no longer have anymore.

What do you mean?

In your start post you say:

I have $20 in my Apple ID balance from a gift card 

That means you have redeem the your Apple Gift Card and the $20 is your Apple Account balance.

When redeeming a gift card, the amount will be credited to your Apple Account balance and you can set your Apple ID balance as your default payment.

How do I get to my apple money to use it I put $20 Apple Cash on my phone how do I get to it?