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”;