In my humble opinion, if you don't know what to do, don't use DevStack. It's for OpenStack developers and testers and assumes you know OpenStack inside-out. While it's an easy way to set up a cloud for exploring OpenStack, it's hard to debug when anything goes wrong. Packstack is a better alternative.

Having said that, check the Neutron server log, usually /opt/stack/logs/q-srv.log (I am not sure what the precise name is, but it starts with q-). Also, I think you haven't switched on DEBUG logging. Do this so that the stack.sh log file contains more information.

You can also read the shell scripts mentioned in the error output, to find out how DevStack determines that Neutron is not running.