Your dogs breath could be smelling like poop because they might have actually eaten poop whether their own poop or some cat poop.
If your dog hasn't eaten any poop or the dog's breath constantly smells like poop then it could be caused by some bacteria in your dogs mouth.
Or the dog could have some condition that is causing it's breath to smell like poop.
You can add some parsley that is cut up to your dogs food or add some apple cider vinegar to the dogs water to help kill off any bacteria that may be causing the dogs breath to smell like poop.
The dog could also have problems with it's intestines that could be causing bacteria to back up into it's mouth causing that poop smell and if so a vet visit would be recommended.
Also brush your dogs teeth using some baking soda and water to make a paste to brush the dogs teeth with and if the smell of poop doesn't go away or keeps coming back and the dog hasn't eaten any poop then I would take the dog to the vet if possible and have it checked out.