The iTunes error 21 - 'The iPhone "iPhone" could not be restored. An unknown error occurred (21)' can be fixed by the following methods:
- Remove the battery from the phone, and leave it without battery for 5 minutes. Now connect the battery and try to restore it via iTunes. The restore will be completed. If still not fixed the Unknown error 21 - move to the next method.
- Try to DRU restore your iPhone to fix iTunes error 21 - You need to put the phone in DFU mode, wait for iTunes to detect it and restore it with the latest iOS firmware.
- Check the host file - Make sure your host file is not blocking Apple's servers.
- Try other computer - Try to restore your device on another computer. This will avoid a software or driver problems.
- Network connection problem - make sure the computer is connected to the internet. Then restore the phone with iTunes.
Hope these methods will help you to fix iTunes error 21 - if still not fixed, please post a comment below.