The warning is because you had a previous cvx_begin without corresponding cvx_end and then another cvx_begin. Everything from the previous cvx_begin is ignored, and CVX will process starting from the 2nd cvx_begin. So it is harmless. You can avoid the warning by issuing the command cvx_clear prior to the 2nd cvx_begin.
As for your original question, please read Why isn't CVX accepting my model? READ THIS FIRST! .