I had the same issue today and solved it by updating Node.js to the latest version.
flatMap was introduced with Node.js v11. You are probably using an older version. You can check it by running: node -v.
I suggest managing node versions using NVM:
Very simple to install with cURL:
curl -o- https://raw.githubusercontent.com/nvm-sh/nvm/v0.37.0/install.sh | bash
Then restart your terminal, and to install the latest Node.js version just run nvm install node.
To check if the Node.js version you are using has changed, you can run node -v again.