Since tense is not marked in Klingon, it is certainly possible that this verb could be interpreted in the past tense. However, you would only do so as part of a complete sentence. With a bare verb like this, we might be asking for the dictionary definition or we might be using it as a complete sentence. Since there is no period in this exercise, we are looking for the dictionary definition. However, since it is easy to miss whether there is a period or not, full sentences are also accepted translations on this exercise. "Walked" is neither the dictionary definition nor a complete sentence, so it is not an accepted answer on this exercise.
However, you would only do so as part of a complete sentence.
Why? I can say the word walked without including it in a complete sentence as easily as I can say the word walk outside a complete sentence. Duolingo doesn't ask for the lexical entry for the word yIt, it asks you to translate the word yIt. Walked is indeed a translation of yIt. So is walks and will walk.