Set the Instant Pot to Sauté and add 1 tsp olive oil. Season the chicken breasts with 1/4 tsp pepper.
When the oil is hot, brown the chicken on both sides, about 2–3 minutes per side. Remove the chicken and set aside.
Add the minced garlic to the pot and cook for about 1 minute until fragrant.
Pour in 2 cups water and deglaze the bottom of the pot, scraping up any browned bits to prevent a burn warning.
Return the chicken breasts to the pot, then add the broken fettuccine noodles in an even layer. Pour the 15 oz jar of garlic Alfredo sauce over the top, ensuring the noodles are covered as much as possible.
Secure the lid and set the valve to Sealing. Cook at High Pressure for 8 minutes.
Quick-release the pressure carefully (a towel can help contain any starchy spray). Let the pot sit 5–10 minutes off heat for the sauce to thicken.
Remove the chicken, slice it, return it to the pot, stir to combine, and serve.