It’s not too much of a literal translation, but I’d say it’s one of the preferred translations when “I can’t tell you” is used in this sense. You can see the Polish expression here as something close to “you can’t even imagine”.
If you translated it literally into “nie mogę ci powiedzieć”, Polish native speakers would understand it as something more like “I’m not allowed to tell you…”. But I understand your issue with this translation, it’s not very helpful in a cloze exercise.
By the way, another idiomatic translation of “I can’t tell you…” would be “nawet nie wiesz…” (lit. “you don’t even know”).