So, I have work on ROS and my understanding about it is much much better ! Thank you for advising me this !
When I try to launch the program takeoff_land I have the error in the previous message.
I have try to follow the response ROS give me but that don't work.
So my problem come from roscore node that doesn't work. When I kill the node and try to start a new one, I have this :
... and so on ...
The log file is empty..
I don't know how made run the roscore. Have you an idea ?
Other question : how long should I wait after requesting OS images on the website ?