"Tune your ʻukulele."
Translation:E hoʻokī ʻoe i kāu ʻukulele.
I have, so far, never been marked wrong for omitting it (though I havenʻt had this particular question yet for English to Hawaiian).
Itʻs possible they just didnʻt have all the variations in the database yet. Theyʻre pretty good about adding them for this course, so that may have changed in the 4 months since you posted.
I think my comment was more about the inconsistency of use instead of correctness. Sometimes DL uses/accepts "`oe" and sometimes it doesn't. This leaves me still wondering whether it's optional? arbitrary? preferred? Hard to know from the results I get here. My kumu says it's proper but not often used in conversation. Maybe the "often" part of that statement is what's driving the variations here. But sometimes I'm marked wrong WITH it, and sometimes I'm marked wrong WITHOUT it. So I never know.