It appears your problem is regarding setting Environment Variables correctly. After installation, the command psql won't work because you need to set the correct PATH to this command in your Environment variables file - which will differ a little according to your version.
I just wanted to add a comment here for future inquiries.
I'm working on a MacOS Catalina 10.15.2 and the current default terminal is using a SHELL called 'zsh'.
I used the following command from the terminal:
and adding the following line:
control X (to quit) and S (to Save) and ENTER (previous filename)
Please note that this path above will depend on your PostgreSQL version - I would recommend you to look this information on your documentation / website.