Did you find this formulation (image) in a book or paper? If so, it is your responsibility to understand it. Does the paper say that Y should be complex? Is B complex? Should you be taking the real part, i.e., minimize(real(B'*Y))? Alternatively, minimize(abs(B'*Y)) would be accepted by CVX, but that is not even equivalent to B'*Y if all variables and input data were real. One way or another, the optimization problem has to have an objective function which evaluates to real.