Trouble changing icloud username

Hello,

Im trying to change my icloud username to something other people will recognize when i send messages from i messages.

This is what shows up when i try and so so

This Apple ID is already in use. If you have an email address ending with @mac.com or @me.com, it is already an Apple ID. To sign in with it, simply enter the email address and password. Or you can create a new Apple ID using a different name.