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You need to update the environment path.
Open terminal.
vim $HOME/.zshrc
Press "I" key for going to insert mode.
add the following line in the opened file:
export PATH="$PATH:/YOUR_FLUTTER_DIR/flutter/bin"
Press "Esc" then write :wq! in terminal and press enter to exit vim.
Reopen the terminal and check "flutter doctor"
If this solution did not work, remove the double quote from path or use the full path explicitly instead of ~.