Round Robin IFR Flight plans require at least one waypoint in the "Route" section of the flight plan form to be accepted. For example if your flight plan is from and to KCPR, put at least one waypoint in the route. A good option is to enter an IAF that you wish to join for the approach at the destination airport.
If you choose to use an airport as a route waypoint, you will need to preface the airport identifier with "DCT", which stands for "Direct. For example, if you are planning a Round Robin IFR flight from KCPR to KCYS and back to KCPR, then in the route field of the flight plan, enter "DCT KCYS" and then in the arrival and departure fields, enter "KCPR."
Note: The use of "DCT should only be used in a flight plan on the File & Brief page. The use of "DCT" in the NavLog on the Maps page will cause "DCT" to be translated as a waypoint instead of meaning "Direct To."