This error was resolved when I disconnected and reconnected my camera module.
Somehow my camera module must have been wiggled loose when I was working with it and the Pi was not able to detect the camera. I ran the command:
vcgencmd get_camera and got Returned supported=1 detected=0
Once the camera was reconnected, I was able to run
raspistill -o image.jpg to test and it worked.
Also, some info I didn't know: The collar on top of the CSI (Camera) port is meant to be lifted slightly and then the camera ribbon is to be inserted and the collar gently locked back down. You shouldn't have to jam your ribbon into the port.