Your hard drive is full. That is what is causing the problem. Once you get the hard drive good and full, the system isn't able to clean up these local snapshots, cache files, "purgeable" files and other things that get lumped into the "System" category. There are various ways that you can clean out these "system" files, but those are always temporary. If you do all of that just so you can download another 12 GB and install a new operating system, I can guarantee you'll have a bad experience. You'll be running out of space all the time.
You haven't said anything about your existing computer or hard drive size. That makes it difficult to make recommendations.
If your current hard drive is less than 500 GB, then you need a new computer. End of story.
If your current hard drive is 500 GB or larger, then you'll need to delete some files. A lot files! No, not those files! Confused yet? You can only delete files that you created, on purpose. Never attempt to delete any files in a hidden directory or a "Library" directory. Don't bother wasting time trying to delete apps. They don't take much space and you'll only cause even worse problems. Don't try using any of those "file finding" apps. Most of them are scams. A few aren't scams, but they will still allow you do find and delete the files that you shouldn't be touching.
Anyway. If you value those files, you should get an external hard drive and copy them off to archive them. Don't use your Time Machine drive. Get a new hard drive. If you don't have a Time Machine drive, then go ahead and get two hard drives - one for Time Machine and one for your archives.
Move the big documents that you probably won't need onto the archive drive. You want to archive at least 200 GB. Otherwise, you'll be right back here with the same problem in a few months. Then delete the originals from your boot drive. And then wait a couple of days for then system to clean up the mess. If you can't find that much data to archive and delete, then go back to step one and get that new computer with a bigger hard drive.
If all else fails, you can erase the hard drive and reinstall a fresh operating system. I recommend turning on all iCloud options first and then just enable iCloud after reinstallation.
And if you were still running Mojave, then your best experience is going to be a new computer regardless.