In attempting to fix the problem, I tried something that, to the best I can recall, no one else has tried. Previously, all attempts to update MS Office Professional 2021 (64-bit) have produced the same error almost everyone else is reporting. This includes updating from within any Office apps, updating via the Command Prompt, attempting to 'repair' from within Control Panel, and updating by downloading from the MS website. In all these attempts, OneDrive was also active (something I've been trying to get rid of).
Part of why I wanted to get rid of OneDrive (besides all the usual reasons people have) was because I installed a second drive in my laptop, and wanted to configure it as the "data" drive (documents, videos, etc). Once OneDrive has been initiated, Microsoft sure makes it difficult to get ride of AND set up a second drive as a "data" drive. Well, I was finally able to get ride of OneDrive AND get everything set for the second drive to be the "data" drive. Once I had completed all of this, and confirmed that everything else related to the laptop was operating correctly, I logged into my MS account, went to my purchased products, and 'installed' Office 2021 over Office 2021. Once this was complete, I discovered that, instead of Build 17726.20126 (what I had prior to this latest attempt), I now had Build 17830.20138 (ie. the most recent update, Aug 1, 2024).
The 'good' news is, I finally have the most recent version. The 'bad' news is, when clicking "Update Now", it still produces the same popup, with the same 30088-27 error message.