Yes :
Inside a given level, just drag and drop an activity

I try it, but if I move a task 'problème shop...' between 'demand d'annulation' and 'Suppression site ...' the system responds that it is not possible to move.

I do not understand why because this is the same project ?

If your print screen if from Gantt Planning view (and not resource planning view), you should be able to move the activity as you describe.

When you move the activity, is the header of the move item "green" or red when you drop it ?

No, the header is red and say that is not possible but i dont know why ?