Turn on a multi-functional pressure cooker (such as Instant Pot®) and select Sauté function. Add oil and allow to heat. Add onion and peppers to hot oil; cook until they begin to get tender, 3 to 4 minutes. Add sausages and cook, breaking them up with a wooden spoon, until lightly browned, 4 to 5 minutes. Add garlic and sauté for 1 to 2 minutes. Add marinara sauce, water, spinach, pasta, basil, and Italian seasoning; mix to combine. Cancel Sauté function.