On the covered Apple iPhone or iPad, 21 means Apple's article lists 21 with the errors that might mean a hardware issue. First, quit and reopen iTunes if it is open, then check third-party security software. Then: Apple files 21 in its hardware group, so update the computer's software or iTunes first. Apple says to retry after each check of the software, cable, computer and network; if the error remains it may be a hardware issue.
Try in this order
0 of 10 completedWhat to do first
ReversibleQuit and reopen iTunes if it is open, then check third-party security software.
Correct only the documented fault domain
ReversibleApple files 21 in its hardware group, so update the computer's software or iTunes first. Apple says to retry after each check of the software, cable, computer and network; if the error remains it may be a hardware issue.
Know which group the error is in
ReversibleApple puts 21 in its hardware group, with 1, 3, 11, 12, 13, 16, 20, 23, 40, 56 and others. That group means the restore failed in a way that can point at the device itself, so the aim is to rule out the computer, the cable and the network first.
Restart the computer and the phone
ReversibleRestart the Mac or PC and force-restart the iPhone or iPad before the next try, since a half-finished earlier attempt can leave both in a state that fails the same way.
Try a different computer
ReversibleIf the same error returns on a second computer with a different cable, the fault is far less likely to be the computer or the cable.
Restore from recovery mode
ReversiblePut the device into recovery mode and restore from there; this is the deeper path Apple uses when a normal update or restore fails.
Book service when it repeats
ReversibleIf error 21 returns on two computers and two cables, Apple says the device may have a hardware issue, so book a repair with Apple or an authorized provider and keep the device unrestored if its data matters.
Enter recovery mode the right way
ReversibleOn an iPhone 8 or later, or an iPad with Face ID, press and release Volume Up, press and release Volume Down, then hold the side or top button until the recovery screen shows a cable and computer. On an iPhone 7, hold Volume Down and the side button together. On an iPhone 6s or earlier, or an iPad with a Home button, hold the Home button and the top or side button together.
Choose Update before Restore
ReversibleWhen the computer offers Update or Restore, pick Update first, because it tries to reinstall the software without erasing your data. Restore wipes the device.
Check warranty coverage before paying
ReversibleEnter the serial number at Apple's coverage page to see whether the device is under warranty or AppleCare, so you know whether a hardware repair would cost anything.
Evidence ledger
Sources behind this answer
Exact questions
Common questions about “Apple iPhone or iPad code 21”
What does 21 mean on a Apple iPhone or iPad?+
Apple's article lists 21 with the errors that might mean a hardware issue.
Does iPhone error 21 always mean broken hardware?+
No. Apple lists it with the errors that might mean a hardware issue, but a bad cable, a security program or a flaky connection produce the same message, so rule those out before paying for a repair.
Should I keep trying the restore over and over?+
Try after each change, not repeatedly with nothing changed. Swap one thing at a time: the cable, the port, the computer, then recovery mode.
Will restoring in recovery mode erase my data?+
A restore erases the device. If you can still use the phone, back it up first; recovery mode offers an update option that keeps data before it offers a restore.