It contains no errors that would seriously undermine the effectiveness of the argument for an educated reader. The author's logic has serious flaws. Syntax or diction may be so flawed that sentences are virtually incomprehensible. In choosing a point the writer has missed the mark entirely. Grammar, punctuation, and spelling errors are such that they make communication impossible.
Plagiarism is also grounds for failure.
A handy guide for learning rhetorical terms. The same, but interactive.