More than enough storage 50.12 GB, but Monterey says not enough?

To reiterate first please review Free up storage space on your Mac - Apple Support and Optimize storage space on your Mac - Apple Support.

If you are still unable to create sufficient space for the upgrade after exhausting all those steps, the workaround is to create a Time Machine backup, completely erase the Mac, install the latest version of macOS (read • below), and finally transfer the contents from that Time Machine backup when asked. To learn how to use Time Machine please read Back up your files with Time Machine on Mac - Apple Support.

• To install the latest macOS that is compatible with your Mac, refer to Other macOS installation options in How to reinstall macOS - Apple Support.

Of course you should not have to resort to such conniptions to upgrade macOS but it is a workaround... since you asked.

Guessing this computer may have the tiny 128 GB SSD Drive - is this correct ?

If yes, how many previous versions of macOS has this computer experienced over its' lifetime ?

Predicated on above as being factual - this is going to be a real challenge to clear enough Empty Space to allow Monterey to install. There after - keeping at least 15%  to 20% of the Drive Total Capacity as Empty Space for good operations of the computer 

You can try the below to try and make the Empty Space.

Hopefully you manage to get the magic number of Empty Space for Monterey to Download ( 12.2 GB approx. ) Expand and then Install.

Rebuild the Spotlight index on your Mac

What is “Other” storage on a Mac, and how can I clean it out?

Free up storage space on your Mac

OmniDiskSweeper Safe to use

GrandPerspective 

How to delete Time Machine snapshots on your Mac

See used and available storage space on your Mac

Locate backups of your iPhone, iPad, and iPod touch

Notation - If the user is using a cloning software like Carbon Copy Cloner - suggest tweaking the Safety Net Feature in this software. It may be making addition Snap Shots that are not being Cloned to the Eternal Drive. If this should be the case, these Snap Shot could be using additional space on the drive 

The final word from Apple on Managing the " Other/ System Data “ Category

Other / System Data: Contains files that don’t fall into the categories listed here. This category primarily includes files and data used by the system, such as log files, caches, VM files, and other runtime system resources. Also included are temporary files, fonts, app support files, and plug-ins. You can't manage the contents of this category. The contents are managed by macOS, and the category varies in size depending on the current state of your Mac.

There can be several reasons responsible for interrupting your Mac’s update process:

1. Lack of free space on Mac.

2. Software malfunction.

3. Internet connection with slow speed.

4. System errors interfere with the updates.

5. Non-responsive update servers.

6. Old install data interfering with updates.

However, you can try the below DIY methods to fix the issue:

1. Ensure sufficient storage space availability.

2. Inspect your Internet connection.

3. Shut down Mac and switch it on

4. Check Apple Update Server

5. Boot your Mac in Safe Mode

6. Remove the older macOS install data

7. Rectify the Software Update preferences

Hope it helps!

Are those 50GB "available" or "free"?

If it says "available", and the "free" space is less, then there is the issue.

Unfortunately, Monterey does not let go of the "available" but not "free" space as easily as it should.

Dlawn wrote:

It started off having ~25GB of available storage with 16GB purgeable storage. As I trashed more files the purgeable amount of storage continuously rises to match the available so that there is always about 17GB more of storage necessary no matter how much trashing of files I do. Nothing seems to change the delta between the available /purgeable amounts.

That means you had less than 10 GB free. That is insufficient to download and install an update. Eventually the operating system will remove those files that you flagged for deletion, but it may take a day or so. But even so, you are critically low on free storage. This can lead to an unrecoverable failure. I recommend keeping at least 100 GB free (not “available”). Use iCloud Drive options and/or get an external hard drive to archive files. Do not use your Time Machine drive. Time Machine is a great backup, but it is not an archive. If you delete files, they will eventually be removed from your Time Machine backup too.

Dlawn wrote:

Unable to update to Monterey even though I have plenty of storage. Any workarounds??

Free up storage:

Free up storage space on your Mac - Apple Support

Dlawn wrote:

Unable to update to Monterey even though I have plenty of storage. Any workarounds??

You need to free up more storage. Monterey is lying about that 50 GB.