@Michelle1225 I would never allow a guest to check into a place which hadn't been cleaned, even if you offer to refund the cleaning fee- that's just asking for a bad review, and isn't fair to the guest.
You say you were "under the impression" that you had a day before for prep- have you checked your settings to make sure that didn't somehow get unchecked? If it still says 1 day before and after for prep time, contact Airbnb to ask them how a guest was able to book that check-in day and ask them how they suggest to deal with it, as you can't possibly get the place ready in time.
If it was an error on your part in your settings, I'd message the guest to explain what happened, apologize profusely, and ask if they could possibly check in at 6 instead, even if they leave their luggage at 3, and offer them an appropriate discount for the inconvenience.