why NaOH connot be used as a primary standard?

A primary standard should essentially available in pure form , stable towards light and heat and react in a stoichiometric proportion.

Sodium hydroxide cannot be used as a primary standard because of it's hygroscopic properties as a solid. Because it is so prone to absorbing moisture, it is impossible to accurately measure the mass of a solid sample in open air.

Also NaOH absorbs carbon dioxide from the atmosphere.