The argument of inv_pos must be concave (or affine). Have you proven your problem is convex? You didn’t show constraints, you showed expression assignments; so we don’t know what the actual constraint you want is, and therefore don’t know if the constraint is convex. For instance 1/norm(...) >= 2 can be rewritten as 2*norm(...) <= 1, which CVX will accept if the argument of norm is affine.