Thanks for using Apple Support Communities.
Since your screen is flashing you may want to look at the hardware troubleshooting article. The error 21 is a hardware issue and is addressed in the quote from the second article.
iPhone: Hardware troubleshooting
Resolve iOS update and restore errors
Check for hardware issues
Related errors: 1, 3, 10, 11, 12, 13, 14, 16, 20, 21, 23, 26, 27, 28, 29, 34, 35, 36, 37, 40, 1000, 1002, 1004, 1011, 1012, 1014, 1667, or 1669.
These errors mean that your device or computer may have a hardware issue that's preventing the update or restore from completing.
- Check that your security software and settings aren't preventing 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.
- Confirm 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.
Have a nice day,