That error message signifies that the battery, which holds power that sustains user data in memory, is quite low, and will need to be replaced. The data it preserves in memory, amounting to possibly as many as 90 waypoints and routes, may be lost if the battery is not replaced soon.
Another tip about this message: in some Garmin GPS devices, this error message means that you need to change the normal power batteries, which charge up the memory backup battery. After 24 hours, the message should go away, if your memory backup battery is OK and gets charged from the normal power batteries. If the message does not go away in 24 hours after the main batteries are replaced, then you need to replace the memory backup battery, which can be done in a service center for Garmin devices.
Would you want to try to replace that battery yourself, if the error does not go away?
Is your setup indicating some other diagnosis than the one I just explained? if so, please Reply To Expert and let me know.