As Hebrew doesn't have use for "a/an" there isn't an accurate translation. You could say כלב הוא רע where כלב is a name and not a noun (same as טרוריסט הוא רע = A terrorist is bad), or in the meaning that dog (any dog) is inherently bad, but neither sound very natural to me. Or you could say זה כלב רע = This is a bad dog.
What would you mean by "A dog is bad"?
Lots of languages don't have an indefinite determiner, but they can still predicate something of an indefinite subject. The sentence is certainly not one I think I have ever heard, but that is the case with many Duolingo sentences, since they are entirely out of context. I always try to think of a context in my head. If, for instance, you were to ask me my advice on the proper pet, I could say, "A cat is good, since it cleans up for itself. A dog is bad." There I am not talking about a particular dog, nor am I saying anything about dogs in all situations, but rather talking about "a" hypothetical dog. It sounds, though, as if כלב הוא רע is precisely what I was looking for. Thanks.