Installation of macOS Monterey never completed

Old Toad’s advice worked. Starting my MacBook Pro With Intel silicon in Safe Mode SOLVED THE PROBLEM namely inability to complete a boot following the upgrade to 12.01. It was necessary to login a second time (after an initial boot in safe mode) which involved a 5 minute reload of the OS. After all that, ABout this Mac shows the OS as 12.01

I've had better luck from the Safe Mode because I evidently have some 3rd party software that interferes with the installation so try the following:

#1 - boot into Safe Mode according to Start up your Mac in safe mode - Apple Support.

NOTE: Safe Mode boot can take up to 3-5 minutes as it's doing the following; 

• Verifies your startup disk and attempts to repair directory issues, if needed

• Loads only required kernel extensions (prevents 3rd party kernel/extensions from loading)

• Prevents Startup Items and Login Items from opening automatically

• Disables user-installed fonts 

• Deletes font caches, kernel cache, and other system cache files

#2 - go to the System/Software Update preference pane and begin the download.

#3 - run the installer from the Safe Mode.

Force shut down by holding down the power key until it shuts down. Power up and see if the installation will complete.

If it doesn't then you may be running into what P. Phillips was referring to.

That is ok - these things happen

Point # 1A - There are reports appearing on the ASC Forums regarding failed installation of Monterey on older Qualifying Apple Computers. In most cases this is related to having a NON Apple Original internal drive ?  The EFI Boot ROM included in Monterey is verifying for Original Apple Drive and if not found , may Balk or fail to upgrade to Monterey. Reports indicate the EFI BOOT ROM is Required to Flash the Computer and is a  requirement for a Successful Monterey Installation.

How much Empty Space was on the Drive before attempting the upGrade to Monterey ?

Anything less than approximately 36 GB is iffy at best and 50 GB Empty Space would have been safer

Macbook pro installation of Catalina 12.2 won't complete. Hangs at about 1/4 done. Forcing shutdown and reboot hangs again at exactly the same point.

Already done that, it resumes the installation and hangs again at the same point.

I did not check remaining disk space before attempting the update. So maybe it was low. Seems my next step may be to boot from an external startup disk??

Interesting as there is no Catalina 12.2 and never been one. Catalina is macOS 10.15 and Monterey is macOS 12 and the released version has only reached Monterey 12.0.1

That OS release number I gave was my mistake. I was installing 12.01 when this happened.