Guys. I spent hours to figure this out. I found a solution.
If you still face this problem after you re-install node, watchman and expo cli etc, You gotta try this simple step. Trust me. Its all bcuz of catalina grant access to folders permission.

First, Kill all the server running and close your terminal.
Go to 'System preference' -> 'Security & Privacy' -> scroll down and click 'Full Disk Access' -> Make sure you checked on 'Terminal' and 'Watchman'. Save and restart ur laptop.

You are welcome.

https://github.com/facebook/watchman/issues/751