How to Fix Error 21 on an iPad

Make sure you're logged in to your computer with an administrative account. If you're not certain, log out and then log in with an admin account.
Update to the latest version of iTunes on your computer by clicking the iTunes 'Help' menu and selecting 'Check for Updates.' If you don't see the Help menu, press 'Ctrl-B' on the keyboard to reveal the Menu bar.
Update Windows. If you don't have Windows' default automatic updates enabled, install any updates manually. Press the 'Windows' key from the Start screen, select 'Settings and then 'Change PC Settings.' Select 'Update and Recovery,' followed by 'Windows Update.' Click 'Check Now.' If there is an update available, follow the onscreen prompts to install it.
Confirm that any security software you installed on your computer is up to date. Windows security software is updated when you do a Windows update.
Unplug all USB devices connected to your computer except for the keyboard and mouse. Examine the iPad's Lightning cable to ensure it is in good repair and then plug it in to a USB port you know is working properly on the computer .
Restart the computer and the iPad. Launch iTunes and try to update or restore the iPad.
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