Error while compiling ElevatorControl: 'IOEntity' does not contain a definition for 'SetFlag' and no accessible extension method 'SetFlag' accepting a first argument of type 'IOEntity' could be found (are you missing a using directive or an assembly reference?) | Line: 69, Pos: 33

Change
//IOChild.SetFlag (BaseEntity.Flags.Reserved8, true); 

to
using (var scope = IOChild.StartSetFlags(BaseEntity.FlagsUpdateMode.SendNetworkUpdate))
{
       scope.Set(BaseEntity.Flags.Reserved8, true);
}