Minimize pos function in frobenius norm

According to cvx user’s guide the function pos(x) is convex but I am not sure either it’s still convex if we apply the frobenius norm.

Here is my problem in cvx

cvx_begin sdp
variables A(5,3)

and it returns the following error:

Disciplined convex programming error:
    Cannot perform the operation norm( {convex}, 2 )

I know that the h=f\circ g is convex if the hessian of g is positive semidefinite and in my example the hessian of pos(A*X+B)-Y with respect to A I am not sure about its semidefiniteness.

No, this is not convex.