@Ruth132 What you do is move on. A critical comment on your sofa-bed is not a violation of the Content Policy so there's no reason for Airbnb to censor it. Star ratings are always subjective, but they're quickly diluted by other ratings if this particular guest's experience was an outlier.
There's nothing fair about a star rating system with no clarity or consistency about what the ratings actually mean. But if the guest was genuinely uncomfortable in the sofabed, I wouldn't dismiss the feedback outright.