gives me error 21

Bruno, since you do not know the history of this phone, anything is possible. the very first thing I would do is to try to plug the phone into an AC charger. It will supply a higher amperage than your computers USB. Clean the dock connector and use a known good cable. If that does not help, replace the battery and while you are at it, check the logic board for any damage and corrosion. Error 21 can occur when security software interferes with the restore and update process, but in your case, it does sound initially like a power issue. Is it fixable, probably. Hope this helps, good luck.