Updating office and getting an error message (Office for Windows)

I have the same problem with Office Pro Plus 2016 when using Outlook. I've tried the fix using Control Panel->Programs->Programs and Features-> MS Office -> change -> quick/online repair but this is temporary. After 1-2 days the update splash screen will reappear and I get again the same error.

I found a different temporary "fix" that doesn't involve constant "repair" of the installation nor restarting the computer. Open the Task Manager, go to Services tab and Stop the "ClickToRunSvc" service. This will kill the "OfficeClickToRun.exe" process which seems to be responsible with the update splash screen. I could then open the Outlook app, go to File --> Office Account --> Office Updates and manually update the product. It shows now "Version 2102 (Build 13801.20294 Click-to-Run)". Still this doesn't seem to fix it permanently as the update splash screen (and the associated error) reappears after a day or two, but maybe MS can have a look into what this service is doing wrong when trying to update the Office products.

Same issue as described above (2/24/21)

First time I tried this method it didn't work, even after restarting computer :(

Tried it again the exact same way a second time (restarted computer) and it worked! Not sure why it took multiple times, but this method did end up working for me.

its not an unusual problem. i have been having it for months. i ended up buying a new 365 subscription to try and fix it since I was still running Office 2016 and it worked that day but then the problem came back again. hours and hours of wasted time and frustration. i thought MS only messed up the Apps they bought, like Skype. now even their own are unreliable and annoying.

See https://support.microsoft.com/en-us/help/2770432/how-to-revert-to-an-earlier-version-of-office-2013-or-office-2016-clic.

Note that reverting to an older release shouldn't be necessary, though. Slowness is not a known issue in recent builds/versions of Word, as far as I am aware. Further troubleshooting may be required.

The Repair feature fixed it immediately.  And NOW it wants a product key.  I have 5 days to enter it. MS office 365 was already installed on this computer when I bought it so I have NO IDEA where the product key is.  Arrrgggghhhh.  I don't suppose there is a way to retrieve it out of the system files? 

Preinstalled versions of Office are usually trial versions. Unless the retailer explicitly said that you purchased a full version, a trial version is what you have.