Hmm. I don't know how advanced and realistic the panel is, but on the 737NG you typically get that message if you:
1.Shut off the engine hydraulic pumps (A and B).
2. Disengage the yaw damper
3. Shut off the flight control systems (A and B). By doing this you cut off all the hydraulic pressure (isolation valve) from elevators, ailerons, rudder, yaw damper and other important functions  . The valve prevents hyd. fluid to enter the systems even if the hyd. pumps are on,

All of these switches are found on the overhead panel btw.

So it seems to be an issue with the hydraulic systems.
I think that the yaw damper  might be the reason. The other systems are fairly advanced and as far as I know only available on  payware aircrafts.

I don't think it's necessary to go deeper into this subject (Hydraulic systems on the 737NG) since it probably would bore you to death