The Swedish and English versions don't match up, and it's sometimes difficult to know when one should give a literal translation and when one should provide a more "natural" sounding one.
In this case, if the "correct" answer is to not be a literal translation then the current "correct" doesn't sound very natural. "I had not thought of that" is probably more how I would say it, or "I had not given it a/any thought".
I'm actually unclear on the usage of this sentence. Is the meaning:
- I hadn't thought of that (used in a situation where someone present an new idea which you hadn't considered)
- I thought no such thing/I wouldn't think of it (used in a situation where you want to make it clear that you did not have a particular thought and as such had no intention of doing something.)
- Joe: Hey Bob, if your internet isn't working, try restarting your modem.
Bob: Oh ya! I hadn't thought of that!
Joe: I saw you looking at my cookie. You were thinking of eating it!
- Bob: No! I thought no such thing!
It's just not idiomatic Swedish, but we accept both "not a" and "no" when translating to English. Someone has turned off the "translate into Swedish" option for this exercise - I suspect it's for this very reason. It's a very good phrase to know, so I don't want to remove it entirely, but it's certainly not easy to teach well on Duolingo.