Amazon account - help please to use gift card

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I had a gift card for £40 which I put on to my Amazon account, which now shows £40 in credit.
I want to buy something using it but it asks for gift card number, which I threw away as thought I didn't need it as balance was on account.
Does anyone know how I can use my credit balance to buy something without having to give a gift card number?
Thank you.

Is it showing in your balance?
When you to to pay does it not default to using the credit balance as first payment option?

Yes, if I go into my account balance, it is there.
When I go to pay, the first option is my credit card, then I can go into gift card, it wants me to apply a code. I no longer have the card as I thought once the balance was on, I wouldn't need it again.

When it offers payment click on where the credit card shows and it should give the option of using the gift card balance instead

If you click on gift card it thinks you have another one to use

Is this purchase you are trying to make more than the credit you have already logged?

This is what shows when I try to buy:
Add a credit or debit cardAmazon accepts all major credit cards.

Your available balance
Use your £40.00 gift card balance
Use £15.99 of your £40.00 gift card balance
Add a gift card, promotion code or voucher

Enter Code
Apply
Use this payment method

You only need to add code if you’re trying to use a new voucher. There should be a little button next to the ‘use £40 gift card balance’ option - just tick that

You don't need to enter the code' just pick "use available balance". Although I think it does this automatically.

Surely you choose the option that uses part of your gift card balance

Use £15.99 of your £40.00 gift card balance

So if your item is £15.99 then click that.

I've done it! I obviously wasn't doing it correctly but got there in the end with your help.
Thank you all

It usually just does it automatically. Can you try just buying it and see if it works?