the NVLD pulls a vacuum on the system overnight and watches for state of change ( aka loss of vacuum)) to ck for small leaks

ck ALL the hoses closely
most common ones are at the purge solenoid @ pass side of the radiator support
ck the hoses at the main body line to throttle body ( both ends) @ pass side engine compartment under the air cleaner hose

ck both the vapor line and vacuum line at the NVLD ( natural vacuum leak detection) pump . @ inside on the drivers side frame rail just rearward of the front suspension cradle

if all is good chances are you need one or the other either the NVLD( most likely with this fault) or the purge solenoid. IF your 100% sure all hoses are good, you might get away with just clearing the fault and immediately taking the car to get ck'd and tags.