"Yukata" robes are sleepwear, but you can wear them outside of your room as long as you stay in the ryokan inn interior. The inside of the ryokan is a place for guests to relax, so you are invited to make yourself comfortable in a yukata robe and slippers. In general, it is not accepted to go outside of the ryokan in a yukata robe, but if the ryokan is located in a hot spring resort area ("onsengai"), sometimes it is ok to walk around outside in a yukata robe. Make sure to check with the ryokan staff.