I had the same answer. "Are you able to" is equivalent to "can you" so should be accepted. Native English speakers have probably heard the story of the 7 year old student who asks his teacher "Can I go to the bathroom Mrs. Smith?" She answers "I don't know if you are able to go to the bathroom or not. Do you mean may you go to the bathroom?" The teacher is teaching that can do something is equivalent to the ability or capacity to do something, not the permission. In short, "able to" should be accepted.
Imperfect verbs are used for regularly recurring activity: "I can get up late on Sundays (this week / often / every day / etc.)" -> "вставать".
If the activity is single, you should be more specific: "I have to get up early next Monday (tomorrow)" -> "встать".
PS: it's probably funny but: "What time are you going to get up tomorrow?" -> "Во сколько ты собираешься завтра вставать (встать)?" - you can use both of them or "Во сколько ты будешь вставать завтра?" - this is quite specific, but we use an imperfect form because of the verb "быть".
PPS: GGMU! :)