Possible Cause

WARNING: This page is about a different car, the 2018 Nissan Titan XD and 2018 Nissan Titan. However, it is still accessible from the selected car via links, so may be relevant.

Malfunction of differential lock control unit power supply circuit (open or short)
Battery power supply
Internal malfunction of differential lock control unit
When battery is less than 6.5 V at cranking, P18CD may be recorded
However, no impact to vehicle behavior will result. (Differential lock system will operate)