Final answer:
The error in the sentence is a run-on sentence, specifically a comma splice, which occurs when two independent clauses are incorrectly joined by a comma.
Step-by-step explanation:
The error in the text "I knocked on the door really loudly, nobody was in." is a run-on sentence, specifically a comma splice. This error occurs when two independent clauses are joined together by a comma without an appropriate conjunction or proper punctuation. To correct the sentence, it could be divided into two separate sentences, the comma could be replaced with a semicolon, or a conjunction could be added. For example:
- I knocked on the door really loudly; nobody was in.
- I knocked on the door really loudly, but nobody was in.
- I knocked on the door really loudly. Nobody was in.