Answer:
The program is most likely run as Viewer. After the installation, there should be two icons on your desktop: Dlubal RFEM 5.23 and Dlubal RFEM 5.23 Viewer. Run the program using the Dlubal RFEM 5.23 icon, without the Viewer extension.
The second reason might be that you have exceeded the number of elements allowed for the demo version. The demo version can contain at most 12 members, 2 surfaces, 2 patterns, or 2 solids to be able to calculate and save a model.