As previous users have noted, the culprit is usually Windows Update. CMD "powercfg -requests" shows this to be the only issue:
EXECUTION:
[PROCESS] \Device\HarddiskVolume3\Windows\System32\MoUsoCoreWorker.exe
USO Worker
Before fixing it, Windows Update show no updates in the queue, but restarting the Update service then shows there is an update ready to download. Today it was the Adobe Flash removal tool, but previously there were other Quality updates stuck in the pipeline.
A constant issue since "Feature update to Windows 10, version 20H2".
Microsoft...PLEASE FIX!
Same with me. Flash removal tool this time. Usually, it's the previews for .NET that gum up the works. I wish MS employees read this board. My feedback report is "MoUsoCoreWorker.exe holding 3 machines awake when optional .NET update comes out" on Feedback hub. Please upvote and post your own.
In the meantime, stopping and restarting the Windows Update service, and then clicking retry in the Settings app always works for me as a workaround, but it will install any optional update without asking in resolving the issue.