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.
A comma splice occurs when two complete sentences are incorrectly joined with just a comma. In the example above, no such comma exists. Madison believed that the best job in the universe would be to work as a crew member on the starship Enterprise is one sentence. Since this job existed only on television, Madison settled … begins the second sentence.
Because there is only empty space between the two sentences, you should call this error a fused sentence.
To fix the problem above, you could put a period after Enterprise and begin since with a capital S.