Cannot use rosbash in Robomaker

I'm sorry you're having issues. Can you tell me more information? Are you running in the Cloud9 IDE? Did you source the setup file for Melodic like this: source /opt/ros/melodic/setup.bash

If you have done that, can you please point me to the documentation steps you completed and where you are stuck?

You're right. Sourcing the setup.bash was all that was needed. Thank you!
I created a new development environment with ROS Melodic as the ROS version and everything else as defaults. What confused me earlier was that the bashrc was sourcing the setup file in /opt/ros/melodic. It sourced setup.sh which doesn't work for some reason. I changed it to setup.bash and that did the trick. Now I don't have to manually source it.
Thanks again!