I just try the conveyor Oliver sent you and it works fine on my end.
You need to run the "Main".
Also, make sure that your Python is set properly. ("Tools"->"Options"->"Python"->"Python interpreter"->"Select").
Also, make sure that you are using the latest version of RoboDK. ("Help"->"Check for updates")
@Olivier, do you know if it is possible to stop the object for at a predetermined location along path? E.g. I would like to stop an object at a position along a curved conveyor so a cobot can attach parts to it, before continuing along the path when assembly is complete.