Translation:Are you free on Saturday?
"Will you be available on Saturday?" should be accepted but did not work. It means the same thing, despite seeming to be different in verb tense.
空 is first tone or 4th tone? When I hear it individually, it sounds like first tone, but in the sentence, it sounds like 4th tone. When I paste this character into google translate to see the pinyin, it shows 4th tone.
Also, "Are you available (on) Saturday" should work.
Sometimes they focus too much on 'meaning' translation, making it harder to test. "Are you free on Saturday" is not a direct translation. It should be "Do you have free time on Saturday". In this case, a direct translation would be preferred and easier to understand for english speakers.