The PMDG has an custom electrical simulation that we'd rather not touch, and in any case in MSFS is not possible to keep the battery recharged by connecting a jetway (it was in P3D, but in MSFS the battery voltage variable is not writable), so that message means, because you can't use the jetway to recharge the airplane, you should use the airplane-own GPU/Jetway power option from the FMC.