Invalid Target for Sequence Flow

[dstoicof]

Hello,

I'm on version 15.1.  While using BPMN 2 to create a workflow I get the error "Invalid Target for Sequence Flow" when I try to connect an outcome of a gateway to a message using the Sequence Flow connector.  It works fine for the other leg (EndEvent).   I can only connect the two using the Association connector.  Is there a guide that explains when to use Sequence Flow and when Association connectors?

Thank you!

[KP]

The specification is here: https://www.omg.org/bpmn/index.htm

There are probably tutorials scattered around the web.