servesbytesnet@raspberrypi:~/byteball-hub $ node start.js
-su: node: command not found
Either
node is not in your PATH, which I think is very unlikely as it should be installed in /usr/bin which is generally included in PATH or
node is not named
node 
but something else, like
nodejs ... See if you have a binary named /usr/bin/nodejs (or something similar) instead of /usr/bin/node. In this case, you might set a symbolic link named
node in /usr/bin to
nodejs.