This is a problem with the process definition.  

When an activity gives you this error, you must go to the activity with the error and click on the Pins tab on the properties for the activity. The default is that all predecessors must fire before the activity can fire. However, if you have a decision point before the activity, it is possible that the decision could end up not firing on the incoming pin for the activity. So, you have to change the Pins property so that it says "Any" instead of "All."