I’d say your problem as written is not very simple, and its convexity seems dubious. I will presume it is non-convex until proven otherwise.
It appears that you are trying to take the reciprocal of a norm, which is not an allowed operation under CVX’;s DCP rules, because it may not be convex or concave. And then sum these. I don’t know whether there is a reformulation possible in compliance with CVX’s rules, but I am very skeptical; and this is not possible if the problem is not convex.
You can not use log2(cvx_expression). Use log(cvx_expression)/log(2)
You should not be declaring expression', and then setting the symbol declared an expression equal to a numerical value vector). If you do, the numerical assignment overrides the earlier expression declaration, and the symbol has the numerical value (vector) assigned, the same as if you never declared that symbol as an expression.