How were the settlers anticipating feeding themselves? I would assume that seeds and farm animals would come along for the ride; even if the planets were habitable, there is no way to know what plants and animals live there, or if they are even edible; however, without the anticipated crop and ranch land, the animals and seeds have to be kept in cryo-storage until habitable domes are set up.
That said, synthesizing food is possible, though not easy, and it's not a good long-term plan. The human body needs a lot of different nutrients to survive, and synthetic sources aren't always as good for you as natural ones.
Without water, there's no point in synthesizing anything, because no one will live long enough to see it. Hopefully, the population has enough water to survive, and a way to recycle waste; there may occasionally be water to find in space, but there is no guarantee. If there's no water, then the population is dead.
Micronutrients are required in small amounts; the inorganic nutrients would be easy to either find or synthesize: Boron, Cobalt, Chromium, Copper, Fluoride, Iodine, Iron, Manganese, Molybdenum, Selenium, and Zinc. Any of those not found locally could be even be pulled from parts of the ship; with requirements in the microgram to milligram range, supplies would last quite a while, and generally could be extracted from waste. Organic nutrients (vitamins A, B complex, C, D, E, K, and Carotenoids) are harder to make, but not impossible, especially if you have some basic building blocks and the right equipment. Certain vitamins (especially vitamin D) are created naturally in the body; other vitamins like C are quite easy to synthesize. Many of the B complex vitamins, however, would be fairly difficult to synthesize, and unfortunately are required for healthy living. Difficult, but not impossible. Luckily, the human body can go without many micronutrients for months, even years, with only minimal health impact. Small amounts of micronutrients, far below the recommended values, would be enough to sustain a population of adults for quite a while. Children, of course, should get as much as possible, as their developing bodies need more nutrients than adults.
Macronutrients are needed in much greater amounts: carbohydrates, proteins, fats, fibers, and alcohols, the basic sources of calories, as well as essential structures like fatty acids. Of those, carbohydrates and alcohols are very easy to synthesize; both have simple structures that can be created in a lab, though it is much, much easier to distill them from organic sources like plants. Synthetic fiber is fairly easy to create as well, and only needed in low amounts, especially with a synthesized diet designed for minimal waste. Synthetic fats have fairly simple molecular chains and could be synthesized. Synthetic protein, however, has yet to be created in a lab, except by cloning - which, frankly, might be the best option. Cloned meat would be much easier to create than chemically-produced food, and the cloning process would use much, much less material. Assuming there are farm animals, samples could be taken from still-frozen creatures for cloning.
While bacteria isn't technically a nutrient, it is required to break down food into a digestible form. On Earth, helpful bacteria is easy to come by, but in space, in a sterile environment with no access to other living beings, it's possible for the bacteria in your gut to go extinct - especially if everyone takes any medication that kills off bacteria. I expect one of the biologists will be a "bacteria rancher", keeping a healthy population of gut bacteria in a maintainable way. Otherwise, all it takes is one round antibacterials to wreck your digestion process forever.
Many nutrients can be ignored for quite a while; not taking any vitamins for a week (or a month) won't kill you, though going for months on end will make you sick. Most people can go without food for a month without dying, and living on reduced calories for several months would have no lasting side effects apart from weight loss. Synthesized nutrients can replace "real" food for even longer, but be aware that synthetic sources of food often have reduced impacts - that is, eating natural vitamins from a plant will have more impact than synthetic vitamins. Frequent, mandatory checkups can help lessen any nutrient deficiencies.
The best case is that the worldship anticipated a "ramp-up" period before the first crops came in, and kept a large stock of pill-form micronutrients and desiccated macronutrients to tide the settlers over. In that case, there would be enough to time clone protein and synthesize the other nutrients, and hopefully get some plants growing. Even basic fungus would be an enormous help towards synthesizing many nutrients.
While it may be possible to sustain a population indefinitely on cloned meat and synthetic vitamins, the population will begin to develop various diseases and disorders over time. They will need to focus on building a suitable long-term habitat; assuming they can find a planet with suitable gravity, they should be able to set up habitation domes within a few weeks, have viable gardens within a year. With enough room, in a few years they should have enough herd animals to start eating real meat. Life will still be tough, but at least they won't starve.