Lentils will be soft and tender, and easily mashed with a fork when they are done. If they haven't reached this texture, place the top back onto the instant pot and add more pressure cooking time. Or turn on the saute button and simmer, stirring often. It is not safe to eat undercooked lentils. You will have to add more cooking time if they're still hard or gritty, until you have really soft lentils. If you're adding cooking time, stir and add a bit of water if needed.