Tomato sauce burns in the instant pot because the sealing ring isn't properly sealed, it is too thick or because there are bits on the bottom of the pot. Avoiding the burn message can be done by making sure that all bits are scraped up from the saute of the onions, garlic and tomato paste. You can also add in 1/4 cup of water or chicken broth to the sauce prior to pressure cooking to thin it and hope to avoid the burn warning.