Catalina 10.15.5 won't install on my macbook air 2014 even though I have 11GB of space

11GB of space is not nearly enough to do the installation.

Actually, it is not enough of free space just for your mac to run day to day stuff, since the OS needs to use some disk space for caches, virtual memory, etc. One is advised to always keep at least 15-20% free on any drive, especially a system drive.

But when updating it needs to expand the installer files, and also keep copies of the old stuff until it finishes putting the new stuff in place.