Humans do not produce Vitamin C due to a mutation in the GULO (gulonolactone oxidase) gene, which results in the inability to synthesize the protein. Normal GULO is an enzyme that catalyzes the reaction of D-glucuronolactone with oxygen to L-xylo-hex-3-gulonolactone. This then spontaneously forms Ascorbic Acid (Vitamin C). However, without the GULO enzyme, no vitamin C is produced. Other mammals like guinea pigs, bats, and dry-nosed primates have gained the same defect. It seems odd that a deleterious random mutation would become dominant in the human lineage. Is that what happened, or was there a sinister manipulation of the human genome?

Your intuition yet again is serving you superbly. The mutation making it impossible for humans to synthesize vitamin C was a deliberate manipulation by the interlopers early along in human history, wanting to make trouble and impair human function. They have done many such manipulations, often on an experimental basis, and capitalize on the genetics program being done by the alien Greys for their own purposes to do the leg-work and introduce something deleterious deliberately.