Closing punctuation (sometimes called end punctuation) consists of periods, question marks, and exclamation points. The most common closing punctuation error is missing punctuation. For example, here, Grammarly has identified a possible issue with closing punctuation:
The student has written,
The results were that emotions affect learning, but strong emotions cannot act as a motive
Since there is no end punctuation, it is possible that this sentence is an incomplete sentence or that the student simply forgot to insert the closing punctuation. To fix the error, the student should write,
The results were that emotions affect learning, but strong emotions cannot act as a motive.
More resources on general punctuation use are at Punctuation.