Answer: The sentence part that is missing from the fragment is the complete predicate.
Step-by-step explanation: A complete predicate includes the main verb or verb phrase of a sentence and the modifiers. Therefore, unlike a simple predicate, a complete predicate contains the words that accompany the main verb/verb phrase. "Owls from the barn" is considered a fragment because it is an incomplete sentence, that is to say a sentence with an important element missing: the complete predicate. "Owls from the barn" does not contain a main verb nor modifiers.