Hephaestus, I really don’t know what you are doing, if you run the Real thunder branch it has all of the main workbenches installed, Fem is one of them. It doesn’t ask for librarys or dependencies or anything else. You can create a bracket in Part Design, switch to Fem, create a mesh surface with either the Netgen mesher or Gmsh mesher.
So back to linux style - you need to know which distribution to get? You're right I didn't get realthunder - downloaded the primary version from freecadweb.org... You'd still think basic functionality would be included in the primary distribution.