You need to edit you Zsh or bash profile. macOS Catalina uses the Z shell by default, so edit $HOME/.zshrc.
If you are using a different shell, the file path and filename will be different on your machine.
PATH_TO_FLUTTER_GIT_DIRECTORY == Location to your flutter SDK file.
To edit $HOME/.zshrc:
open Terminal
copy and paste nano $HOME/.zshrc
copy next line and paste it at bottom
export PATH="[PATH_TO_FLUTTER_GIT_DIRECTORY]/flutter/bin:$PATH"
Edit your [PATH_TO_FLUTTER_GIT_DIRECTORY].
It will look something like this--> for example:
export PATH="/Users/flutter/bin:$PATH"
press CTRL X and when it asked you to save the file, choose yes.
Close and Restart the Terminal and try running flutter doctor
Verify that the flutter/bin directory is now in your PATH by running:
echo $PATH
[PATH_TO_FLUTTER_GIT_DIRECTORY] is where you installed flutter SDK.
Instead of nano, you can use any text editor to edit ~/.bash_profile or .zshrc