While there can probably be many reasons for this error, here’s one that I had:

msgNew() = “http://MySite.com/MyApp/MyService.svc”;

There was not value in the parentheses. For some reason, this does NOT put the red exclamation point on the Message Assignment shape.

 

I had been testing the following, then changed it, and forgot and left it blank

msgNew(BTS.OutboundTransportLocation) = “http://MySite.com/MyApp/MyService.svc”;