Easier just to do it on another Mac or PC. Make sure your iTunes is up to date too.
It's because you're trying or have tried to jailbreak.
Google iTunes error 21 and you'll get suggestions.
I did then it says could not
I have the same issue basically, my ipod tells me error 40 in itunes and i've tried every method to get it out of recovery loop and it WILL NOT. Does your updatelog say something about failed to initialize NAND or failure communicating with NAND? If so you may be in the same boat as me!