Yes, you can cook short ribs from frozen in the Instant Pot. Just note that they won't develop as much flavor this way, because you will have to skip the browning step. Browning the short ribs before cooking them adds a tremendous amount of flavor, so I really don't recommend skipping this step, but if you're really strapped for time and don't have time to defrost the ribs beforehand, don't let that stop you. You'll need to add an additional 10 minutes to the cook time when using frozen meat.