When you're in the Listing Editor, can you see whether you've entered the number of guests? It's just below Availability. Maybe that's the "capacity" it's looking for?
I think @Shelley159 is correct. Sounds like you haven't published the listing yet. Use the back button to go all the way back to the beginning of the setup and you should see a page where you add the number of guests. Be sure that is complete.