When your Mac says it's almost full but there is plenty of available space, you need to check the purgeable space.
The available space on Mac isn't the actual free space but a combination of purgeable space and free space. When the free space is low, you'll receive the alert "Your disk is almost full."
Purgeable space usually consists of Time Machine local snapshots and other files that temporarily hold free disk space but should be emptied when your Mac is nearly full. But in real life, your system may not release the space when you need it, as in your case.
To truly free up your Mac, you can use iBoysoft DiskGeeker to clean the unwanted files first. This disk cleaner can help you easily locate and delete large files and junk files that you no longer need. Click the button below to give it a try!
Then follow the steps in the guide below to delete the Time Machine local snapshots.