Hi E-J,
First thing first, congratulations for the event, most appropriate for me as for the moment I can only fly GA aircraft using X-Plane 11. Why nothing else works for me any longer is a long story and completely unrelated to the topic

So I checked all stops along the route in X-Plane 11 and I met a couple of problems.
The first is one you already answered, about the non-existing ETSM. I even looked for it on maps and could not find it, but if it's the same as EDJA then problem solved!
The other one is ETEB which exists in X-Plane 11 (not sure about earlier versions), but looks like a helicopter air base with an appropriately short runway (it even has "H"s marked along it). This one is ~1200ft long, which is too short for the Baron, at least in theory

I would suggest EDQF nearby, with a grass runway but one of a more appropriate length of ~2000ft. (I am not sure but this may still be too short for the Beech1900. EDFR has a longer asphalt runway but is further away).
On the good side... ETEB is the last stop before completing the event so it will be some time to decide before the leg opens.
Andrei