Since the recent update to Mac OS Catalina and forced update to zsh, I was having problem with a lot of my CLI tools, including flutter. The reason being, zsh uses ~/.zshrc and not ~/.bash_profile
So to solve the problem, simply add your flutter path to ~/.zshrc like this
export PATH="$HOME/your_path/flutter/bin:$PATH"
Change your_path to where you had flutter downloaded and extracted on the earlier version of Mac OS.