How can I reclaim 80.9 GB of unusable storage on Mac?

I think you're misunderstanding - or, at least, misstating - the problem.

You don't have "80GB of unusable storage" on your drive.

What you have is 80Gb of storage that the OS hasn't (yet) categorized into the other buckets.

There is a question, for sure, as to why it's struggling to categorize this data, but the fact is this space is not 'lost' or 'missing', it's being used by something, it just isn't yet clear what's using it.

If System Settings is struggling to categorize the data, you might try something like OmniDiskSweeper which uses a different approach of showing you the files/directories on your system without trying to categorize them. At the very least it should point to which files are taking up the space and you can then make an informed decision as to what to do (e.g. if it is your old photos that are also in your Photos Library, then you can delete them).

Meme_Lord387 wrote:

[...]

Like I said, is there anything else you can think of that I could try?

MacOS may save a number of Time Machine snapshots on the drive as part of the backup process.  TM snapshots can occupy a significant amount of drive space. You can manage these snapshots and possibly recover some storage space.

If you have enabled TM to back up the computer automatically and you don't keep the backup drive connected at all times, then TM will create these TM snapshots that it will use at those times when you reconnect the backup drive.

You can disable the creation of these many TM snapshots by setting TM to backup manually rather than automatically at intervals. Or you can keep the backup drive connected to the laptop at all times.

You can view and manage these in the Disk Utility app. There is the potential to recover many gigabytes of storage. View APFS snapshots in Disk Utility on Mac - Apple Support

Just to build on what @ D.I. Johnson has rightfully outlined

According to your image presented

The Internal Drive has a Capacity that is almost full

The computer is too busy trying to find Empty Space to save changes that are being made

Reducing System/Volume/Data is a common question. 

1 -  System data taking too much in MacOS Sono… - Apple Community

2 - Time Machine Local Snapshot won't delete - Apple Community

3 - Over 60% storage blocked by System Data - Apple Community

How to free up ‘System Data’ and other storage on your Mac from a fellow colleague  @ neuroanatomist

Use another application to see where space is being used  Storeograph  on the Apple Apps Store 

Suggest getting an External SSD Drive and start moving your Pictures, Videos, Music, Messages and any other large files you have control over, OFF the Internal drive and Onto the External

Understanding iCloud Drive from a well written User Tip from @ Richard.Taylor

There are  two effective ways to remedy this issue:

1. Quick Fix Action as previous mentioned by @ D. I. Johnson

For Apple Silicon computers, use Disk Utility to erase a Mac.

For Apple Intel computers, use Disk Utility to erase an Intel-based Mac, then reinstall macOS.

Always make a Time Machine backup before proceeding.

Migrate only the user account, not the entire system.

Reinstall only the necessary applications from the Apple App Store or directly from the developers.

2 - Generally

When the user discovers this issue,

it’s likely because the computer’s internal drive capacity is small, such as 256 GB

Unfortunately, the user’s storage needs may have increased since the computer was purchased.

To future-proof the computer, consider spending extra money upfront on a larger drive capacity and adding more unified RAM.

Note - On Apple Silicon and newer computers.

The SSD Drive and the Unified RAM are Soldered to the Logicboard and can not be upgraded.

To get some addition space and predicated on you image in above

At that time when the image was posted

Messages application with a File Attachment like Pictures, Videos, Music

It is using about the same amount of Space and Used Space in your installed Applications

To get rid of the file attachments in Messages

System Information >> General >> Messages

Click to the right side were is list the used space

There is a circle with a I ( eye ) inside of it

Click it

You may be give and Pop Up that looks like in below

Choose the files ( all )

At the bottom use the Delete button

You should get “clean my mac” it just does magic or try checking your disk space using terminal where you’ll get the real data the terminal never lies

You're fighting a losing battle, my friend. You'll be playing 'whack-a-mole' until you replace that Mac.

The cause of the issue is simply that your startup disk is much too short on capacity. While 256 GB may be fine for an iPhone or iPad, it is not suitable for any computer.

About the only thing you can do is start over by completely wiping the drive and reinstalling a fresh copy of macOS.

Even then, your gain probably won't last.

There simply is no other reliable way to reclaim an appreciable amount of storage space on a drive that small.

mendel176 wrote:

You should get “clean my mac” it just does magic or try checking your disk space using terminal where you’ll get the real data the terminal never lies

Not a good suggestion

Any Third Party Applications that will interfere with the normal operation of the OS,  is an invitation for disaster. 

Certain Applications maybe available on the Apple Apps Store - this only means the Developer is prepared to pay Apple a percentage on each sale. 

What the Application may do to the computer is up to the User to check this out before purchase

Any of the below should be removed as per Developers Instructions 

Disk Cleaner 

CleanMyMac , aka “ BrickMyMac “

To put the Third Party Disk Cleaner / Optimizer in context and the damages it may have, will do or has already done.

This type of applications can or will Muck Up your User Account ( Home Folder ) of this machine.

It does not touch the Operating System itself unless you consider your User Account ( Home Folder ) as part of the Operating System.

The Operating System may be  hosed

It seems I linked to a related but not-quite-the-right Apple Support article previously. Apologies to all.

I meant to include this link directly related to Time Machine local snapshots: About Time Machine local snapshots on Mac - Apple Support

Actually, both D.I. Johnson and Owl-53 have provided you with "what you can do" in this case. Whether or not you got this laptop as a gift, its limited internal storage space will require more diligence on your part to not run into this issue.

Their suggestion to basically "start over" with a "clean" system, is a good start and I suggest that you consider doing so. Using an external drive, also suggested, will make a significant difference with this laptop's useability.

Thank you for mention 👍

🦉-53

One month since the issues was first raised

The Internal drive will not have increased in capacity in that month

It will require actioning on the users part