Answer: The blue sneakers were Queen Elsa's favorite; they
were also the most expensive item in the store. or
The blue sneakers were Queen Elsa's favorite, but they
were also the most expensive item in the store. or
The blue sneakers were Queen Elsa's favorite. They
were also the most expensive item in the store.
Step-by-step explanation:
The error in the original sentence is called a "comma splice." You have two independent clauses. As they are closely connected, you may use a semicolon between the clauses, or you may use a comma and a coordinating conjunction.
Most commonly, use a period and a capital letter to make two separate sentences.