Answer: It has no subject.
Explanation:
It should be "I did the entire puzzle in less than an hour." Or "Steve did the entire puzzle in less than an hour." Or something like that. It needs a pronoun or a name at the beginning, otherwise it has no subject and it's not complete.
You could also make it work if it's connected with another sentence that has a subject. For example- "I've already finished- did the entire puzzle in less than an hour."
