It is generally a good computer practice to alway keep at least 20% to 25% of the Total Drive Capacity’s as Empty Space.
Allowing the computer to drop below these guidelines may eventually cause unintended consequences such as the Alert it has already presented.
There is Purgeable Space and there is Empty Space.
They are Not one and the same thing
Purgeable Space which is Controlled by the Operating System.
When the Operating Systems decides the computer needs additional Empty Space, it will move a portion of the Purgeable to Empty space
AFAIK - there is no User Actions to hasten this transition from Purgeable to Empty Space
It can day or longer before this will occur.
The links below will help in identifying what is taking up space on the Internal Drive.
It will also provide possible ways to remove data that is under the direct control of the User ( Home Folder ) .
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
GrandPerspective
How to delete Time Machine snapshots on your Mac.
Often caused if the Time Machine Drive has not been attached to the computer and TM Backup is set to run on a Schedule.
TM Backup will make Snap Shots on the Internal Drive awaiting the TM Backup Drive to be attached.
Only then, will the Snaps Shots be transferred to the External Drive.
View APFS snapshots in Disk Utility on Mac - Apple Support (CA)
See used and available storage space on your Mac
Locate backups of your iPhone, iPad, and iPod touch
If there a Suite of Adobe Applications used on this computer ?
They may create some very large cache files that can be removed. Though, the Adobe cache files will be recreated as the Applications needs them.
https://helpx.adobe.com/ca/premiere-pro/kb/clear-cache.html
The same clearing of System Cache files can be achieved by booting into Safe Mode
They will be recreated as the System Requires
