The most likely cause is that your hard drive is full. When macOS tells you how much "available" disk space you have, it is flat-out lying to you. You see, Apple has chosen to use the term "available" which doesn't mean what you think it means. Apple's definition of "available" is free storage space plus any allocated storage that "could be" make free at some point, if Apple, not you, decided that it was needed. This calculation is out of your control. It is Apple that decides when you can delete files and when you can't. All you can do is ask Apple to delete your files. Then Apple will delete them on its own schedule, if it feels the need.
I say all of this because when I tell people their hard drive is full, they inevitably try to delete files. Then they get even more confused when the more they delete files, the less storage they have. What???
I don't have enough information to say why you are out of disk space, only that you are. Depending on your particular computer's configuration, deleting more files could 1) correct the problem, 2) make the problem worse, 3) corrupt your system to the point of requiring a complete reinstall. I don't know which will happen.
To add insult to injury, if you try to use Apple's storage management tools, you might get even more confused.
You can use 3rd party storage tools. You just have to be very careful with them. Most of them will show you storage all across your system. In most cases, you should never attempt to manually delete files in under any "Library" folder. This is how you corrupt your system.
You also have to be careful with your iCloud settings. If your internal hard drive is smaller than your iCloud storage space, then you MUST use the "Optimize Mac Storage" setting. Otherwise, you could get to the point where deleting files causes you to lose storage, not regain it.
Good luck!
