Preheat oven to 400° F and line a large baking sheet with parchment paper.
Toss the sweet potatoes with olive oil, chili powder, salt and pepper, and spread onto the baking sheet. Roast for 20 minutes, or until golden brown.
Make the avocado yogurt sauce: In a small food processor, combine the yogurt, avocado, garlic, lime juice, and a few generous pinches of salt and pepper. Pulse until smooth. Taste and adjust seasonings. Chill until ready to use.
Assemble the tacos with a scoop of the sauce, the roasted sweet potatoes, black beans, and desired toppings. Season with salt, pepper, and squeezes of lime.