Only one start event should be modeled in a diagram. If a process has more than one start event, it may not be clear.
The clear definition of one start event is important because it behaves as the process trigger. When using multiple start events, it may not be clear when a process is to begin. There should only be one start event in a BPMN diagram.
If you receive this error, make sure all event types are used correctly in your diagram.