Cold sores are caused by virus.

Describe why a virus such as the cold sore virus cannot be cultured on a nutrient agar plate

Viruses in general require a living host cell for replication. So cold sore virus requires a livng host to replicate. Infected host cells can be cultured and grown, and then the growth medium can be harvested as a source of virus as compared to bacteria that can be cultured in a nutrient agar plate. So ther virus must have a host cell like bacteria, plant or animal in which to live and make more viruses. Outside of a host cell, viruses cannot function.