Here is the sentence again:
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.
Sima whined that she didn't have the right pencil is a main clause. At the same time, she realized ... is the beginning of the next main clause. These two parts are joined with no punctuation, causing a fused sentence.
A period after pencil and a capital A for at would take care of the problem.