Here is the item again:
Struggling to capture the nude model on paper, Sima whined that she did
not have the right pencil at the same time,
she realized that this complaint was the same as saying that she could not
sink a basketball because she did not have expensive athletic shoes.
Sima whined that she did not 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.