Why is my puppy throwing up green fluid, won't walk, and lethargic?

What a sweet face. I'm so sorry that Nala is sick. You need to get her to a vet ASAP. If she is unable to hold down water and she's very lethargic, a number of serious problems are possible. She may have a viral enteritis (Parvovirus), or an intestinal obstruction from a toy, rock, etc. Many causes are possible but the bottom line is that if she is lethargic and cannot hold down fluids, then at the very least she needs to have some help getting some hydration. Please seek an emergency vet as soon as you can. They can do an exam and see if she is painful in her abdomen, they may recommend a Parvo test, they may recommend blood work to check her electrolytes and blood sugar, and they may recommend X-rays, all depending on what their impressions are from physical exam and the information you give them. (vaccine history, any known toys she's chewed up lately, when did the vomiting start, is she having diarrhea as well, etc etc. ). They can give her fluids either intravenously (IV) or under her skin (sub-Q fluids, also called subcutaneous fluids) and can advise you on the next best steps for Nala.

I am sorry I don't have a home solution for you here, but you really need to get her in to your vet ASAP. A lethargic puppy is never normal. (Tired after playing hard, yes. Lethargic, no.)

I hope thats helpful, good luck with her. She's a beautiful girl.