Upgrade to Monterey - An error occurred loading the update

As you've tried Safe Mode already, you can try repairing the disk - How to repair a Mac disk with Disk Utility - Apple Support

You could also try reinstalling macOS with macOS Recovery - How to reinstall macOS - Apple Support

If neither of those work, you may have to erase it and restore form a backup - Use Disk Utility to erase an Intel-based Mac - Apple Support

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.

After some looking, it seems Monterey simply isn't compatible with HDDs, only SSDs. If your 2019 iMac doesn't have an SSD, you may be out of luck.

Additionally, Monterey can't be installed on the non-original HD.

From here - MacOS Monterey Won’t Install Onto Macs wi… - Apple Community

HWTech wrote:

The Monterey installer also appears to require an internal original Apple OEM SSD as an additional requirement.

I'm sorry I couldn't be of more help.

Is the User Profile accurate iMac 27″, macOS 11.2 ?

If yes, there is probably the issue.

Attempting to UpGrade from Big Sur 11.2 to Monterey may not work.

Suggest updating the current version of Big Sir to Current version 11.6.7

Then, attempt the Monterey upGrade again.

___Bubba___ wrote:

After some looking, it seems Monterey simply isn't compatible with HDDs, only SSDs. If your 2019 iMac doesn't have an SSD, you may be out of luck.

Additionally, Monterey can't be installed on the non-original HD.

This is not correct actually. A few weeks ago I upgraded a late 2015 iMac from Sierra (10.12) through 12.4 Monterey and it had a rotational mechanical HDD, not an SSD. That said, the performance of an APFS rotational HDD with Monterey is rather poor, but it certainly can be done and these Macs are listed as compatible with Monterey on Apple's Monterey System Requirements page.

Monterey can be installed on a non-original HD. Because the above 2015 iMac was so slow with the original HDD, we removed the HDD and installed a Samsung internal SSD and then installed Monterey (12.4) on that drive. It now works great, is very fast.

The issue with the original Apple drives and Monterey is that with SOME Mac models, apparently an original Apple drive is required to be connected to complete firmware updates that come along with the MacOS update/upgrade. The Apple Installer will sometimes refuse to install the MacOS updates if only a non-Apple drive is there and the firmware has not already been updated with the Apple drive in the past.

User wrote " Monterey can be installed on a non-original HD. Because the above 2015 iMac was so slow with the original HDD, we removed the HDD and installed a Samsung internal SSD and then installed Monterey (12.4) on that drive. It now works great, is very fast. "

Q - Was Monterey first installed to the Rotational equipped Computer & then the Rotational HD was replaced with the SSD ?

Where upon Monterey was installed o the After Market SSD.

Asking as all reports appearing on the Monterey Forums specifically mention the Original Drive must be An Apple and confirmed by multiple source .

Once the EFI BOOT ROM has Flashed the Computer, it allow the Replacement of Original Apple Drive with an Aftermarket Drive

Perhaps the SSD is the key and Rotational Drives are exempt ?

Of course- glad I could be of some help.

You could of course contact Apple Support for help- if you have a Genius Bar or similar close by, you may be able to take it there.

Try booting into Safe Mode and do both: download and run the installer from there.

ChrisVivid wrote:

I hope I don't need a boot drive?

No, you won't- after the reformatting, your Mac will start up in Recovery Mode to guide you through reinstalling macOS. Good luck!