Express does not support Assemblies as it has told you so unless you can get a hold of a full version of Simulation you will not be able to analyze the assembly. However, you can try saving the assembly as a part and then run your analysis that way.
The Xpress simulation only works on single components, not assemblies. Take the part you want to investigate and apply the resulting forces on that single part. i.e. make a small surface on the bolt where each plate touches it and apply the forces there.
You need one of the actual simulation packages to simulate assemblies of mutliple bodies and have it resolve the internal forces.