Take a look at the sentence again:
At the campus coffee cart, Gini makes the best
drinks her sweet cream latte, a blend of vanilla ice cream and espresso,
will put on the pounds, but its cool, smooth taste is worth a trip to the
gym.
The problem occurs between drinks and her. Gini makes the best drinks is a main clause, so it can stand alone as a complete sentence. Her sweet cream latte, a blend of vanilla ice cream and espresso, will put on the pounds ... begins the next complete sentence. Because these two parts have only blank space between them, they make a fused sentence, a major sentence error.
To fix the problem, you could put a period after drinks and then capitalize the H for her.