Usually this error occurs when the start activity node is not set to start which is by default in ‘Normal’ state. Here are the steps to set the property of ‘Start’ node.
Right click on the activity –> Go to Properties –> Select ‘Node’ Tab
As shown in the above screen shot you will find that Start/End field is set to ‘Normal’ which is the default value. Change the value to ‘Start’ from LOV as shown in the below screen shot.
You will find small difference in ‘Start’ node icon which will have a green color arrow mark are left side bottom of the node.