Translation:It is not necessarily an expensive suit.
This is funny to me. I'm imagining that someone is telling me to steal the suit.
"It isn't necessarily an expensive suit" should also be accepted.. :(
It should, but try to make it a habit to avoid using contractions on Duolingo. It confuses its poor computer brain, especially since "is" and "has" is contracted the same way.
"Det" alone can mean "that" depending on context, though you generally put more emphasis on it than if you mean "it". I suppose if I had to choose I'd accept it, but with the side note that "it" sounds a lot more natural to me in this particular context.