Upgrade to 11.4

[Parallels User]

each time I upgrade my OS X guest OS's using 11.3 or greater, the install hangs at about the 40% mark. Then I 'stop' the guest, restart it, and send the data on a failed boot to Apple, and the system boots fine. Guess this is just an FYI, as after the stop and restart, the install finishes (11.4)

 

[Gimbal]

It's a bug, but you can temporarily modify one line in a file, perform the guest macOS software update, then revert the file. See here:
https://forum.parallels.com/threads/macos-11-01-not-updating-to-11-2.352462/page-3