Running out of space?
From October 2019
If you get this message it likely means your computer’s storage is above 85% capacity, and you might experience issues with your computer until the drive gets 100% full—at which point your computer becomes unusable. To find out how much storage space you have (macOS 10.12 and later), go to the Apple icon at the very top left of your screen and click “About This Mac” and then select the Storage tab at the top. The Storage window will show a Manage… button. From there you will be able to see which files, folders, and applications are taking up the most space on your drive.
For more complete instructions check out thisarticle on Apple.