You have violated CVX’s GP rules.Geometric programming mode — CVX Users' Guide .You are multiplying b1, which is a difference of two variables, by other variables to produce net1_sum1; hence the error message. Of course there wold be many more error message if CVX ever got that far.
Have you proven your problem is convex (representable by GP mode)?
I will mark this problem as non-convex unless and until you show otherwise.