Again hard to know exactly what you are trying to do, with these bytes.
That is:
0xff 0xff - is the start of a packet
0x00 - ID… Not sure if this is valid ID ? looks like it might be, but I never tried it. Dynamixels ship with ID=1
0x04 - length
0x03 Instruction write
20 - starting register 0x20 32) Moving Speed
FF - first data byte? Moving speed is 2 byte so not sure what will happen
D8 - Checksum? Did not compute it to see if right.
But again not sure what you are trying to do as this is only setting one byte of moving speed? Which is used when you set a goal position.
You might try using one of Robotis libraries to help you out. Something like Dynamixel SDK or Dynamixel Workshop… More information up on their e-manual pages, like: DYNAMIXEL SDK
Note: I don’t use Python so probably much more help here.