Take another look at the sentence:
Struggling to capture the nude model on paper, Sima whined that she didn't have the right pencil at the same time she realized that this complaint was the same as saying that she couldn't sink a basketball because she didn't have expensive athletic shoes.
Struggling to capture the nude model on paper is a present participle phrase. A present participle phrase will begin with an ing word like struggling. Sima whined is a main clause. You must use a comma to connect these two parts.