install mac os

Depending on just how close to the OS telling you the Drive Is Full - you might even need to use Transfer files between two Mac computers using target disk mode to salvage you personal files. The, maybe you will be able to do the Cleaning of the remainder of the drive as per previous suggestions.

For the future - one need to keep at least 10 - 15 GB of Empty Space on the Drive at all times for the OS to function and function properly

If you are really that low on disk space, you're going to have to get about 40GB of disk space before you can upgrade MacOS.

Another option, more extreme but much faster to get to your end-point is to use Time Machine to backup your entire machine, or offload your data to an external disk. Time Machine might be faster.

Then, once your sure your data is backed up, wipe your machine and reinstall MacOS on a clean, empty machine.

It all starts with turning on your computer while holding command-R. Use Disk Utility to erase your Mac's hard drive, and resinstall MacOS. You can then plug your TimeMachine drive in and restore whatever data you want.

https://support.apple.com/en-us/HT204904

See if the following support article can help: How to free up storage space on your Mac - Apple Support

I suggest that you will have to do more to get sufficient free space. Also, depending on how full your HD is, the computer can stop functioning completely. I suggest that it is time to install a storage disk which you can use to transfer files onto in order to clear up the needed space. These days, these external disks are reasonably priced, compact, and very easy to install with provided USB cables. Most of them do not even require external power. You may have to reformat it, though. Easy to do and problem solved.