Best 5 tested professional methods to fix iTunes error 21 on iPhone 5S 5C 5 4S 6 6S & 7 and complete the restore / update process
Here are the methods to fix iTunes Error 21 on your iPhone
1. Restart iTunes Tool
2. Update iTunes App
3. Disable Your Antivirus Software
4. Check Your Host File
5. Unplug All other USB Devices
If still not fixed error 21 problem, follow the link in the description for more tips to resolve this error.