question for help: why can't I have a vertext without an out path in the model? #282

As mentioned in the title,if I have a model as figured bellow.When I run the model to generate test cases,error comes out.The error tells me that no valid path for the vertex in cirrcle.
I wanna to know why can't I have an end vertex in model?Or what's wrong in the model?
Thanks for help,please.