Place a 6-qt Instant Pot on Saute setting then add coconut oil. When oil is hot, add onion, garlic, ginger, and lemongrass (if using); cook 3 to 5 minutes or until tender. Add sweet potatoes and the next five ingredients (through salt) to the pot; stir to combine.
Lock lid in place. Set Instant Pot on High Pressure to cook 8 minutes. Turn off Instant Pot; let stand for 15 minutes to release pressure naturally. Release any remaining pressure. Open lid carefully.
Using a potato masher or fork, mash some of the sweet potatoes and chickpeas to help thicken. Place Instant Pot on Saute setting. Add green beans. Cook, loosely covered, 4 minutes until green beans are tender, stirring occasionally. Add lime juice. Serve with garnishes (if using).