I have had a similar issue. I can’t believe TripAdvisor supports Viator. The customer service sends you in wild circles when all you are trying to do is not be double booked. They have not sent the right information to the touring company and have caused a $150 tour to be completely useless. I contacted them ahead of time and they refuse to do anything at all. It is even a sold out tour. They could easily fill the single seat. Don’t fall for the pretty pictures and the great advertising, Viator has terrible customer service and more or less appears to be a scam. Use Airbnb whenever you can, the experience is entirely opposite and positive.