I want a task executer to move boxs from a place to three other places, so I created a task sequence which includes tasks like travel-load-travel-unload-travel-load-travel-unload-travel-load-travel-unload ,but when the task sequence go on to the second load task , it didn't work and remind me that the task is invalid. I checked the tree and find out that it failed to find something to load but I don't know how to fix it.