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What is Man can not synthesise ascorbic acid due to deficiency of enzyme and explain in detail?
ASCORBIC ACID (VITAMIN C):

The active form of vitamin C is ascorbic acid. 
Its main function is as a reducing agent. 
Vitamin C is a coenzyme in hydroxylation reactions (for example, hydroxylation of prolyl and lysyl residues in collagen) where its role is to keep the iron (Fe) of hydroxylases in the reduced, ferrous (Fe+2) form. 
Thus, vitamin C is required for the maintenance of normal connective tissue as well as for wound healing. Vitamin C also facilitates the absorption of dietary nonheme iron from the intestine by reduction of the ferric form (Fe+3) to Fe+2.
Man and other primates, guinea pigs and bats cannot synthesize ascorbic acid due to the deficiency of a single enzyme namely L-gulonolactone oxidase.

Key Concept:
Man cannot synthesize ascorbic acid due to the deficiency of a single enzyme namely L-gulonolactone oxidase.
Ref : Lippincott’s illustrated reviews for biochemistry , 7th edition.