My experience is that even though this message is referring to "low memory", which should refer to RAM, I've only heard of it appearing when the user's hard drive is perilously close to being full. I know that this is counter to how it should logically be, but those are the facts.
If you have a hard drive that has 964GB free of 1TB total (i.e. your hard drive has hardly been used to store anything), then it sounds like you have something invisible that has filled up your hard drive, yet isn't registering. I've seen this way more than you might expect.
Download one, or more of these (they are all free). They will tell you if something is filling up your hard drive, and if so, what it is:
DiskInventory X (free)
Disk Inventory X
or
GrandPerspective (free)
GrandPerspective
or
OmniDiskSweeper (free)
Creators of Mac, iPad, and iPhone productivity software. Proud to bring you OmniFocus, OmniOutliner, OmniGraffle, and OmniPlan.
www.omnigroup.com
The most common source of such a problem is that TimeMachine has been enabled, but there is no external hard drive turned on or attached so TM is keeping local snapshots of backups on your main drive. That is, when the computer is running, but not connected to its external Time Machine drive, it will try to make TM "snapshots" and save them to the hard drive, then when you re-connect the TM drive, it moves them there.:
Time Machine in Mac OS X Lion initiates a sometimes useful, sometimes not feature: local backups. Called snapshots, this seems to be kicked off when your primary Mac is a laptop and the Time Machin…
osxdaily.com
Employee monitoring software and service - WorkTime
The second possible source of this problem is a huge sleep image:
Learn how to control which type of sleep your fall 2005 (or newer) laptop uses.
www.macworld.com
If you use Dropbox, it may be the culprit. Go to the Dropbox gear wheel which is at the bottom right of the DB drop down menu. In the window that opens, choose Advanced. Next to the title Selective Sync: click on Change Settings. Uncheck all the folders/files you do NOT want automatically synced to your computer. Everything in Dropbox will double up on your computer unless you opt out.
Lastly, I've seen this problem caused by an out-of-control cache or log file. That is, your Mac may be encountering a problem or error, and that same error is being logged over and over, causing a log file to quickly grow until it fills up your hard drive. To see if this is the problem, download and run this to see if the problem temporarily disappears:
Maintence (free)
Maintenance is an operating system maintenance and cleaning utility for macOS that you can use to perform miscellaneous tasks of computer maintenance: run periodic scripts; rebuild the databases; delete application, font, and system caches; and more.
www.titanium-software.fr