Select the "saute" setting on the Instant Pot. Once the Instant Pot reads "HOT" pour the olive oil in the pot and place the sausages inside the Instant Pot. Begin to brown the sausage on all sides for 5 minutes.
While the sausage browns, begin to chop all the vegetables including the garlic.
Once the sausage is golden in color (but not cooked through) remove the sausage and set to the side. Add in the garlic and onions and saute for 30 seconds until fragrant. Then add in the bell peppers. Mix and saute for 1 minute then add in the salt, Italian seasoning, sugar, and black pepper. Mix well then add in the tomato paste and mix until combined.
Add in the diced tomatoes and pureed tomatoes. Mix well and then add the Italian sausage back in to the Instant Pot. Make sure you submerge all the links. Place the lid on the Instant Pot and move the valve to "sealing" position. Turn off the Instant Pot then select "meat/stew", select 15 minutes.
Once the sausage is done, let the Instant Pot naturally rest for 10 minutes. Then, carefully, force the remaining pressure out of the Instant Pot. Mix well, and serve immediately.