Lots of carnivores don't have the ability to manufacture all of the amino acids their body requires. They need to get them from other animals, often because their digestive system isn't able to break down plant matter. The same is true of many vitamins. Each animal is different.
This doesn't mean that it necessarily has to come from animals though. Unlike many animals, humans don't manufacture their own vitamin C. We can get vitamin C by eating other animals, but we can also get it from sources rich in vitamin C like oranges, bananas or lemons[1]. If a human isn't able to digest citrus, it's also possible to obtain vitamin C supplements.
[1] I'll note that limes are lacking enough in vitamin C that when the Royal Navy switched from lemons to limes (they were cheaper) sailors started getting scurvy again, and the 'cure' for scurvy was lost until the actual reason that lemons cure scurvy (vitamin C) was discovered.
