Drizzle 4 1/2 teaspoons olive oil over diced sweet potatoes in a bowl. Add garlic and salt. Toss to coat potatoes in oil. Spread on a rimmed baking sheet in a single layer.
Roast in the preheated oven, stirring every 15 minutes, until tender, about 40 minutes.
Heat remaining 1 1/2 teaspoons olive oil in a skillet over medium heat. Cook and stir onion until translucent, 5 to 10 minutes. Add corn and cook until corn begins to brown, 5 to 8 minutes. Stir in taco sauce, taco seasoning, and coriander. Add water and bring to a simmer; simmer gently until slightly thickened, about 3 minutes.
Stir roasted sweet potatoes and black beans into the skillet to coat with sauce. Spoon mixture over tortillas.