When I set up my Arduino App, I clicked through the setup without really paying attention, assuming that the default serial port it listed was the correct one. I got this message when attempting to "Upload" to my Uno using the wrong Serial Port. I was able to change my Upload port to the correct one by going to [Menu Bar] Tools > Serial Port and selecting the USB port where my device was plugged in.
I know this is a dead topic, but I just figured I would post in case other new users experience this.