'ls' command not working on mac OS Catalina
If you do not want to change your shell from bash to zsh, then add this to your .bashrc file:

export BASH_SILENCE_DEPRECATION_WARNING=1


To edit this you can use this command:

nano ~/.bashrc 


While you are editing your .bashrc file, look for aliases or functions which redefined ls use Ghostscript.

Try which -a ls in Terminal.app to show you what ls is currently defined as, and what other ls commands are available.