Got your the same problem on my PMDG 747. Look at your upper panel IRS, are they properly set? All 3 should be set on NAV. Make sure your IRS is aligned correct (in FMC see this under index POS, do you have 3 positions?). Align the aircraft you can only do on ground when the aircraft standing still. You can also lose your align if you for example lose power for a short while on ground, if you turn off APU or ground power too quickly. If you see this warning in the air you probably need to land manually. And next time do you preflight check-list properly
