A MessageFlow cannot target an EndEvent or an IntermediateThrowEvent, or have a StartEvent or an IntermediateCatchEvent as its source.
A StartEvent begins a sequence and cannot have outgoing message flows. Similarly, an EndEvent ends a sequence and cannot have any incoming message flows.
A CatchEvent node catches messages but cannot send them, while a ThrowEvent sends messages but cannot catch them.
If you have incoming flows on a StartEvent or outgoing flows from an EndEvent, you must remove them. If you have incoming message flows on a ThrowEvent, you should consider changing them to a CatchEvent instead. Similarly, if you have outgoing message flows from a CatchEvent, you should consider making them ThrowEvents instead.