Hi. This means that the expo command is not in your PATH.
Did you install expo-cli with npm or yarn? Based on the output you posted I think npm.
You should be able to see where it was installed by running npm -g bin. On my machine this returns the following:
$ npm -g bin
/usr/local/bin
npm can be set up to install to your home directory instead in which case it might be something like:
/Users/youruser/.npm-global/bin
e.g. here’s another thread where someone else had the same problem: