Heat olive oil in a skillet over medium heat; cook and stir onion and garlic in the hot oil until translucent, 10 to 15 minutes. Stir in peanut butter until melted, 2 to 3 minutes. Mash sweet potatoes into onion mixture; cook and stir until heated through and creamy, about 5 minutes. Fold in cilantro and black beans.
Stack tortillas on a microwave-safe plate and cover with a paper towel; cook in microwave until just warmed, about 30 seconds.
Spread sweet potato mixture onto each tortilla; fold each tortilla around the filling, creating an envelope shape. Top each with a dollop of sour cream.