Since your questions is about homework, we want to share our policy where we state that we can help you find a solution, but not provide one directly
However, in this case the error is not vital for the homework itself I think. The message Object 'tweets_3' not found indicates that the object was not defined higher up in your code and thus does not exist at the time it is used in the code shown. Check all variable names and order of the code to fix this.
Additionally, we recommend creating a reprex if you post here so we can more easily help you out in future. A reprex consists of the minimal code and data needed to recreate the issue/question you're having. You can find instructions how to build and share one here: