Hi tamora102!
First of all, I would advise for you to have a look into this article before changing the Apple ID name: Change your Apple ID - Apple Support
It is very important, for instance, to sign out of all your devices on the multiple services, before changing the Apple ID (if you do, make sure you've got your trusted phone number up to date - in case you use two factor authentication or two step verification - or that you've got full knowledge of your security questions - in case you use a secondary authentication account).
The fact it says your email address is not available at the moment it could mean that it is either an Apple ID already (you can check on iforgot.apple.com) or that you might be using it as a rescue email for another Apple ID.
If you think it shouldn't be any of the cases mentioned before, do reach Apple Support where they will most likely be able to sort out your problem!
Cheers!