Preheat the oven to 190°C (375°F) with a rack in the centre.
If the sweet potato is raw, cube it and microwave in a covered bowl with 2 tbsp of water on high for 5 to 6 minutes until soft, or roast it at the same oven temperature for 20 minutes.
For the sauce, simmer the crushed tomatoes with half the chili powder, cumin, garlic powder and salt in a saucepan over medium-low heat for 5 minutes.
Mash the black beans with the sweet potato and the rest of the spices and salt.
Warm the tortillas for 30 seconds in the microwave, wrapped in a damp towel, so they roll without cracking. Spoon the filling down each, roll up and lay seam side down in a baking dish.
Pour the sauce over the enchiladas, covering the ends so they do not dry out, and sprinkle with the nutritional yeast.
Bake for 20 to 25 minutes, until bubbling at the edges.