You need to arm the vehicle before it will allow you to control the motors. There are also various failsafes that you’ll need to either account for or disable for continuous control.
This example could be a useful reference, and includes some relevant notes at the bottom.
Are you creating a h264-encoded video stream that’s being served at port 4777? If not, you’ll need to either do that or change which port the code is looking at or which encoding it is trying to parse the stream as.