This is an EnergyPlus support group -- it looks like
OpenStudio has not written the Controller:MechanicalVentilation object
correctly or you are using a later version of EnergyPlus than the one
they are writing for.
I suggest that the OpenStudio support group is probably better for your
question.
Or, you can send your idf here or to the helpdesk but we'll just tell you
the Design Specification Outdoor Air Object Name 1 field is
blank.