A google of that error finds priors. https://www.google.com/search?q=iTunes+update+error+message+2721&ie=utf-8&oe=utf-8
What's not consistent is the fix. It could be permissions or an overreaching antivirus. The error itself does not tell us what to do.
Example? https://discussions.apple.com/thread/3177664?tstart=0
It worked for one person but not the next.
Why is this? Your PC is not like the next PC.
I have a Dell XP 8700 desktop computer running Windows 10. I'm trying to update iTunes software and get an error message 2721 at the end of installation. I was told by Apple tech people to uninstall iTunes and reinstall it, I still get the same error message. I contacted Apple again and was told that it is a Windows problem and not Apple's problem. Can anyone help me with this issue?

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