* Monotremes: These are egg-laying mammals like echidnas and platypuses. They don't have nipples, instead, they have mammary glands that release milk through pores on their belly.
* Some species of fish: While not strictly mammals, some fish species are known to have mammary glands and produce milk for their young. These fish lack nipples, however, and release milk through pores.
So, the answer depends on what you consider "nipples." If you define nipples as a specific structure, then monotremes and those fish are the answer. If you're looking for animals that don't have any way to release milk, then the answer would be all non-mammals, including birds, reptiles, amphibians, and insects.