Final answer:
The error in the sentence is a fragment, as it lacks a main verb.
Step-by-step explanation:
The error in the sentence is a fragment. A fragment is a group of words that lacks a subject, a verb, or both, or it is a subordinate clause punctuated as though it were a sentence by itself. In this case, the sentence 'The youth group met at 4:00 p.m. to protest against the proposal for an extended school day' is a fragment because it lacks a main verb.