Matrix solve

So the symbolic in Mathcad 15 has a hard time to calculate the symbolic inverse using your new matrix SP and when it finally manages to do, the result of the single elements are so large that Mathcad refuses to display even when you try to display single elements only (at least that applies to the first three elements).

I guess that Mathcad 11 with Maple as symbolic engine is able to display the results, but I wonder in which way these huge results would be useful for you!  So you either have to use another, more capable software or you rethink about what you actually want to accomplish.
If you later intend to provide values for all your variables, setting up the whole thing as a function in all those variables might be a solution. As the error message says, you can still use the symbolic results in further calculations, Mathcad just refuses to show them.