Need help with simulation!

Good Evening,

I am doing a FEA analysis project and was having some problems with solidworks simulation. I don't quite understand how there is always error in either creation of mesh or with the stability of the structure despite already placing enough fixtures. The structure here is a steel jacket supporting a platform stiffened with stiffeners(extruded grillage), the bracings and 4 main columns are meshed as beam with the platform and the stiffeners and meshed as rigid solids. Naturally i want the analysis to be as realistic as possible but i am not very sure if i am going the right way with my boundary conditions. Right now i am just playing around trying to understand the linear static analysis before moving on to other stuff. Please assist! Really appreciate it!!

SolidworksSimulation