Not everyone can be vegan: food deserts exist
True, and a real constraint for some people. It is also not a description of most of the people making the argument.
Large numbers of people live where the only food within reach comes from convenience stores and fast-food outlets, and they often lack the money, kitchen, storage or spare hours that cooking beans, grains and vegetables from scratch demands. For them animal products are frequently the cheapest, most filling and most available calories, so a moral demand that everyone go vegan ignores the real conditions of poor people's lives. An ethic that only the comfortable can meet looks less like a universal obligation and more like a privilege dressed up as a duty.
Food access is unevenly distributed. There are households without a nearby shop selling fresh food, without reliable cooking facilities, without storage, or without the time that cooking from raw ingredients requires. For them the choice set is genuinely narrower, and veganism's own standard (possible and practicable) recognises exactly this.
The argument becomes a different thing when made by someone with full access. At that point the constraints facing others are being invoked to settle a question about one's own choices, which they have no bearing on. The constraint is real and the inference from it is not.
Introducing a point that is not relevant to the question under discussion, drawing attention away from it rather than answering it.
How it applies here: The constraints facing people without food access do not bear on the choices available to someone who has access. Two different situations are being treated as one.
Replacing someone's actual position with a weaker, distorted version, then refuting that instead. The original argument is left untouched.
How it applies here: Responds to a claim about what is possible and practicable as though it were a demand that everyone everywhere must be vegan immediately.