Make sure that controller.py has the robotID and send_video.py the cameraID. Check if there are any typos and you haven't mixed them up.
Sometimes, the server hasn't assigned a video port for your robot.
Ping @Ged on Discord so he can set it manually.
--type none to when starting the script, like this:
python controller.py <ROBOTID> --type none
Most likely if you are asking this question you have gone somewhere without an HDMI monitor and you can't SSH into your robot because you can't connect it to Wifi.
Be sure that in your robot settings page "mute text to speech" is not toggled on.
Try changing the speaker to a different USB port and turn the robot off and back on.
Try unplugging the camera, turn the robot off and back on, wait for "ok" and then plug camera back in.
You can also try updating and upgrading the Raspberry Pi software. SSH into your robot and do:
sudo apt-get update sudo apt-get dist-upgrade