Can untreated vitamin b12 deficiency cause nerve damage ? if yes,can it be seen through nerve conduction study ?

Yes and yes: Yes solidly objective B12 deficiency (by formal laboratory test) (usually implying pernicious anemia) can cause nerve damage. Yes, it can be seen on electrodiagnostics except at very early stage. Note that in true B12 deficiency taking oral B12 does not work because the cause is a stomach abnormality that prevents B12 absorption. Solidly objective tests can confirm.