Try this stuff
Check for hardware issues
Errors: 1, 3, 11, 12, 13, 14, 16, 20, 21, 23, 26, 27, 28, 29, 34, 35, 36, 37, 40, 53, 56, 1002, 1004, 1011, 1012, 1014, 1667, or 1669.
Your device or computer might have a hardware issue that stops the update or restore from completing.
Check that your security software and settings aren't blocking your device from communicating with the Apple update server.
Then try to restore your iOS device two more times while connected with a cable, computer, and network you know are good.
Make sure that your security software and settings are allowing communication between your device and update servers.
If you still see the error message when you update or restore, contact Apple support.
If you still see the error message, check for outdated or modified software.
1 Comment
Can U tell me what type of hardware issues? Like I'm having all original cables.. So what type of hardware?.. Actually I just..disabled the security policy of my pc..but also after that it is not able to restore my iPhone always showing error 21
by Shashwat Deshmukh