Answer:
Error 108) appears if the stress analysis cannot be performed because the design situation preset for the design has not been defined.
By default, RF‑CONCRETE Surfaces requires a load or a load case from the "characteristic design situation" for the stress analysis of the reinforcing steel. If it has not been defined, the design is not performed and the corresponding error message is displayed.
To avoid this, select a "characteristic load" in the "Serviceability Limit State" tab of Window "1.1 General Data". See the video.
As an alternative, you can change the detailed settings for the serviceability limit state design so that the stress analysis is performed with a quasi-permanent load, for example. However, this change of the default setting or standard specifications is at the discretion of the user.