I Can't, Not Won't 못 vs 안

안 마셔요 sounds like preference, you just don’t drink milk. 못 마셔요 signals an obstacle: you’re lactose intolerant, or it makes you sick. Say 못 먹어요 about a food you simply dislike and it can land strangely, like you’re claiming a physical inability you don’t actually have.

없어서 in that last sentence means “because there are none,” built on 없다 plus a reason ending, -아서/-어서. That’s not the grammar point here, it gets its own note later, but it’s common enough that it’s worth spotting on sight before then.