Review the item:
Madison believed that the best job in the universe would be to work as a
crew member on the starship Enterprise since
this job existed only on television, Madison settled for clerking at a neighborhood
comic book store that sold Star Trek memorabilia.
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 starship 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.