Mathcad has two completely different subscripts You have used the literal index (Lx.i) which is simply part of the variable name, just lowered and smaller. But as you want to create a vector you should have used the vector/matrix index (Lx[i).
They both look very similar but you can notice the difference if you click in the region - then you see the dot in your expressions which shouldn't be there.
If you don't like keyboard shorts - you get the literal subscript from the formatting menu bar as for normal text and the matrix subscript from the Vector and Matrix Toolbar.
