Turn on a multi-functional pressure cooker (such as Instant Pot) and select Saute function. Once pot is hot, add oil, green and red bell peppers, and onion. Cook until vegetables are tender, about 5 minutes. Lightly prick sausage casings with a fork and add to pot. Brown sausages on all sides, about 5 minutes. Add tomato sauce, garlic, Italian seasoning, and salt. Close and lock the lid.

Turn off Saute function and select high pressure according to manufacturer's instructions; set timer for 8 minutes. Allow 10 to 15 minutes for pressure to build.

Release pressure carefully using the quick-release method according to manufacturer's instructions, about 5 minutes. Unlock and remove the lid. Cut sausages on the diagonal into small pieces. Serve immediately.