Error: Process Model Properties at processName: Variables(s) not found

Hi, I'm a newbie going thru the Acme Exercise tutorial. I'm building the AX New Vehicle process model following the directions.  When I try to Save the process model I get the following error:

There was an error in an expression in Process Model Properties at processName: Variable(s) not found: ri!vehicle. 

But everything appears to be in place.  I even deleted the input and re-created it.  What am I missing?