Vitamin C cannot be produced in humans due to lack of:

(a) L-gulonolactone oxidaseRef: Harper's illustrated biochemistry, 30th ed., pg. 200-201In liver, the uronic acid pathway catalyzes the conversion of glucose to glucuronic acid, ascorbic acid (except in human beings and other species for which ascorbate is a vitamin, vitamin C), andpentoses. It is also an alternative oxidative pathway for glucose that, like the pentose phosphate pathway, does not lead to the formation of ATP. Glucuronate is reduced to 1-gulonate, the direct precursor of ascorbate in those animals capable of synthesizing this vitamin, in an NADPH dependent reaction. In human beings and other primates, as well as guinea pigs, bats, and some birds and fishes, ascorbic acid cannot be synthesized because of the absence of l-gulonolactone oxidase.