Turn on a multi-functional pressure cooker (such as Instant Pot); select sauté function and adjust to medium.
Cover the bottom of the pot with avocado oil. Cook and stir onion until soft, 4 to 5 minutes. Add garlic and cook until fragrant, about 30 seconds.
Add rice to the pot; mix until coated with oil and lightly browned.
Pour in chicken stock; stir any browned bits off the bottom of the pot.
Mix in tomato sauce, salt, cumin, and cayenne pepper.
Close and lock the lid. Seal the vent and select high-pressure function. Set a timer for 7 minutes; allow 10 to 15 minutes for pressure to build.
Release pressure carefully using the quick-release method according to the manufacturer's instructions, about 5 minutes. Unlock and remove the lid. Stir rice before serving.