iTunes song credit Help

You don't need to convert it, just find the song that you want and the credit should automatically be used to 'pay' for it : How iTunes Store purchases are billed :

When you buy individual songs from the iTunes Store:

• Free song credits are redeemed first.
• If you have no free song credits, then your gift certificate, prepaid card, and allowance credits are used next.
• If your gift certificate, prepaid card, or allowance credit is less than the total cost of the purchase, your credit card is billed for the remaining balance.
• If you have no free song credits or gift certificates, prepaid cards, or allowance credits, your credit card is billed for the songs you buy.

My account has five song credits on them. When I try and use them my account asks for my credit card number. I don't use a credit card with my account, only gift cards. The five song credits were issues as part of an apology that my account had a complex problem that took a week to fix. Apple did fix the original problem, but now this.

Is this the first attempted purchase on a new computer/device ? If not then since getting the credits from Apple have you tried logging out and back into the account on the computer/device that you are using, and the credits show on the account on that computer/device ?

The devices aren't new and have had no problems with purchases or downloads. I even went out and got a new gift card to see if downloads were an issue. If I have gift card money on the account then downloads go through and the credits don't get used. I have tried logging out of my account and then back in on each device and the same issue continues on.

If the credits aren't being used try contacting iTunes Support and see if they can see why : https://www.apple.com/emea/support/itunes/contact.html