In RFEM 6, you can only design reinforced concrete members and surfaces. To do this, use the Concrete Design add-on. This can perform the ultimate limit state, stability, and serviceability limit state design.
Reinforced concrete solids cannot be designed directly in RFEM 6.
However, it is possible to create solids with the "Concrete" material and determine the stresses within the solid, for example. Optionally, you can add a result beam into the solid, which is used to convert the results of the solid to the member internal forces. You can then design this result beam in the Concrete Design add-on.