Turn the Instant Pot on to the saute function and wait for it to get hot. Season chicken with salt and pepper. Add 1/2 tablespoon oil and the chicken to the pot. Cook 2 to 3 minutes on each side or until browned (chicken will not be done). Remove from pan.

Heat remaining 1/2 tablespoon oil in pot. Add onion and bell pepper, and saute 4 to 5 minutes or until tender. Add garlic, and saute 30 seconds. Stir in marinara sauce, and return chicken to Instant Pot, covering it with sauce.

Cover Instant Pot, and, using manual setting, set to 10 minutes on high pressure, making sure the pressure valve is sealed. When done, quick release the pressure.

While chicken cooks, prepare spaghetti according to package directions. 

Remove chicken from Instant Pot to a bowl. Turn the Instant Pot on to the saute function, and allow sauce to simmer 5 minutes or until thickened. While sauce simmers, shred chicken with 2 forks. Return chicken to sauce, stirring to combine. 

Serve chicken mixture over spaghetti. Top each serving with Parmesan cheese and basil (if using).