Nanotechnology can cause all the right atoms to be arranged in the right way, so it doesn't violate the laws of physics.
Using the assembly routines generally present for the development of living animals, which is the for-sure way to create such an animal (why invent wholescale general replicators when living animals already self-assemble?) will run into some issues.
When growing in a womb, there is a whole adult animal providing working support systems while the embryo is growing, and using active metabolism to provide processed nutrients.
Thinking of an egg instead, you need pre-processed stuff with more than enough mass for the final assembly to be prepared ahead of time, which is nothing like pulling more primitive stuff out of sludge.
You'll also find the egg is limited in size by scaling laws. It can't get enough oxygen for anything larger, for example.
Perhaps looking at the way plants work: a system of roots and vines can grow fruit which are actually "eggs" for the desired creature, but being actively fed and energized they are really "wombs". The idea of roots and already working plant-like tissue can process raw materials, but what about getting enough energy? An autotroph would need a huge amount of solar collector surface area and it would also take a long time as the overall metabolism is slow. Would the one plant even be able to maintain the growing animal fetus?
So look at fungi, not plants. No visible huge leaves, just hidden hyphae "eating" the energy-rich material already present. That's getting somewhere: it's hidden under the surface, it can digest dead animals and other plant matter rather than having to build everything up from air and sunlight. But the rapidity of the metabolism is still not high enough to grow and maintain a warm-blooded animal. It would be more animal-like itself, using circulating blood rather than slowly diffusing protoplasm. It would be animal-like tissue spreading out like roots, maybe like a Siphonophore which is a really cool SF-ish creature!
The animal spreads out colony parts along vine-like tendrils, including mouths/stomachs for gathering food from the bog, gills to get oxygen, and might even have active hunting elements that remain tethered or return to the sedentary bodies. Think of jellyfish parts. And one huge bell is actually a womb with the bioengineered solderer being grown inside.
There's your answer. And it's scary and awesome in its own right!