Not Enough Space to Install Update on MacBook Pro

Being available does not necessarily mean being free, to check the available space on your drive, select it

in the Finder and press Command-I to open the Info window.

The Info window will show you the amount of space that is free and the amount that is considered "purgeable."

Ideally, all the available space should be utilized when needed, but macOS tends to hold onto it, a lot of this space may be reserved is through the creation of "local snapshots" by Time Machine. So, you can temporarily disable automatic backups to allow local snapshots to be deleted.

Once you remove old unwanted Downloads, you may still have space problem.

I suggest that most users have already removed files as aggressively as they would like to, and chasing down more will only provide minimal relief, and only for a very short time.

The real solution to the problem of having more files than you have space is to add more space -- by connecting another external drive.