The statement which triggered the error has y*x, which is the product of two variables, which violates CVX’s rules. You can not enter the problem directly as you have shown it. I don’t know whether it is possible to reformulate to be enterable into CVX, even if we give you the benefit of the doubt that it really is concave. I suggest you re-read Appendix B, whatever that is, and study the proof of concavity. And look at the rest of the paper or book to see what it suggests about using the Lagrange dual method.