Is there a W, Q, V or H fare filed on that route (at your time of search) that allows for one way travel or meets other conditions (day of week, etc.). Doesn't matter what fare code shows as open, If there is not a fare filed that meets the travel conditions then there is nothing to purchase.

As others have suggested, you can try the same route as a round-trip or on a different day to see if those fares are valid then.