Cvx operation problem

There is my part of function codes.

d= norm(X, 2);

Where X= Ax-b is a convex function’s input parameter:
A is 44 matrix
n= 4
b is 4*1 matrix

variable x(n) ;
minimize( sum(myFunction(A*x-b)) );

Then cvx compiler return:
Error using cvx/norm (line 52)
Disciplined convex programming error:
Cannot perform the operation norm( {convex}, 1 )

Error in myFunction(line 3)
d= norm(X, 2);

It’s perplexing me long time.
Thanks for your response.

Please read the documentation fully, in particular the section on the DCP ruleset, and the section on compositions. Thenorm function is convex and non-monotonic, so its argument must be affine, and you are passing it a convex argument. In all likelihood your problem is not convex.