Hi Steban. Your problem resides in fluid not being created. For what it's seen in your screenshot it seems that water is correctly defined and added. Did you Save and generate GenCase before executing? (In the dropdown Save menu, Clicking on Save and execute GenCase).
If it's right and you did it, check out the vtk files created by GenCase with a software like paraview to see how your case is transformed into particles. You should find the case without water, based on that error. I don't know the reason why, though.
BTW, I'm not the mantainer of DesignSPHysics anymore, but I'll keep helping over here whenever is possible.