Why doesn t NaCl conduct electricity in a solid state?

There are two particles which can conduct electricity : they are mobile ions and delocalized electrons.

All electrons are moving, but only delocalized electrons (sometimes known as free electrons) can conduct electricity. Delocalized electrons are electrons that delocalized between atoms or ions, but not localized in a particular atom or ion. Only metals and alloys contains delocalized electrons.

Sodium chloride contains sodium and chloride ions. In solid state, the sodium and chloride ions are not mobile, and all electrons are localized in sodium or chloride ions. In the absence of mobile ions and delocalized electrons, sodium chloride cannot conduct electricity in solid state.