you're getting the error because you forgot to index B,
C2(i) = B(i)*log2(1+min(i))
But anyway, what do you even mean by min(i), i in your case is just a scalar.
I'd recommend you to rename the loop varible , i, to something else since i is the default imaginary unit in matlab. Nextly, pre-allocate A, B, C2 and d properly. For example,
A = zeros(1,length(ro_db))
TIP: when you see read underlines on your code, it means matlab is trying to help you by giving some warning and even recommendations, do not ignore them.
