Take a look at the sentence again:
The problem occurs between Enterprise and since. Madison believed that the best job in the universe would be to work as a crew member on the star ship Enterprise is a complete sentence. Since this job existed only on television, Madison settled … 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 Enterprise and then capitalize the S for since.