Change the event to "A unit starts the effect of an ability".
If that does not work, try adding a debug message action to see if the trigger is even running at all. You might have duplicate Conscript abilities and are checking for the wrong one. Also, I'm not sure if the ability itself has filters to cast only on villagers, but atm that trigger would turn any unit the spell is casted on into a halabadier. Just a heads up.