In Unix systems such as Linux and MacOS, you need root (administrator) privileges in order to write files to the root of the file system (i.e. the folder
In short, you should save files somewhere inside of your home directory and not in the root of the file system; that would be
/home/quinzer on Linux or
/Users/quinzer on MacOS (replace
quinzer with whatever your user name actually is).
The save dialog for your system should have a button or some such somewhere that lets you jump to that location. It's usually a little house, a picture of a house on a folder, etc.