Set Instant pot to sauté function. When the display reads "Hot" add the garlic butter. When the butter is completely melted add the chicken.
Cook chicken, stirring often, until no longer pink outside. The chicken should not be cooked all the way through.
Pour Alfredo sauce over the chicken. Add chicken broth, roasted red peppers, and pasta to instant pot. If you are using frozen spinach, you can add it now. If using fresh spinach, do NOT add it yet. Stir well to combine.
Secure the lid and set the valve to sealing. Press "Manual" and set the timer for 10 minutes. When the timer beeps, allow pressure to release naturally for 15 minutes. Then quick release the remaining pressure by turning the valve to venting.
When the pressure valve has dropped indicating that all of the pressure has been released, carefully remove the lid. Add fresh spinach and cheese. Stir in and replace the lid.
Wait about 10 minutes for the cheese to melt and spinach to wilt. Serve immediately.