How to empty or clean up system data

The category named System Data (or Other) is a general category that measures the storage space used by all Apple and third-party files that don’t belong to any more specific category like Applications, Documents, Messages, Music Creation, Podcasts, Trash, and TV.

You can’t directly manage the contents of system data. That is done by macOS. The category varies in size depending on the current state of your Mac.

What you can do to create more free space:

• Reboot your Mac at least weekly. Let the OS clear caches and do housekeeping.

• Empty the Trash in the Dock. 

• Empty the trash in the Photos app.

• Delete unused and unneeded application installers from your downloads folder and desktop. No need to store on your Mac what you can freely download any time.

• Transfer files that you don’t use daily to an external drive and then delete them from the startup drive and empty the trash. Files that take up the most room are movies, images and music.

This from Apple on the subject of freeing up space:

Free up storage space on Mac - Apple Support

Also, please see the following guidance from Apple: 

Find and delete files on your Mac - Apple Support

If this does not work for your, then you could take the 'scorched Earth' approach and

Erase your Mac and reset it to factory settings - Apple Support

This wipes everything and should not be done if you don't have your stuff backed up on another drive.

we are seeing users of recent versions of macOS

who have enabled Time Machine, but

do not connect their backup drive often enough --

... sometimes get a lot of pre-staged backup 'snapshots' on the boot drive. These snapshots contain the lists of files (but NOT copies of the files themselves) to be backed up when the external drive is connected again.

If this is impacting you, the fix is to connect your external drive and let Time Machine make the copies it wants onto the external drive. After that, it will remove the snapshots automatically.

If that does not solve the lack of FREE space, you may have to use a Utility to find the largest files. several Utilities are mentioned in this article:

How to free up ‘System Data’ and other st… - Apple Community

"Cleaner" apps and App deleters are some of the most egregious crap-ware out there. Don't even THINK about using an app deleter to solve this issue -- they will only give you multiple problems instead of the simple one you are facing now.