Hi Alvaro,
Try to change FUB to
FUB = ListPlot[{Transpose[{x[NN], AQ}], Transpose[{x[NN], CU}]}, Filling -> {2 -> {1}}, PlotMarkers -> {{"\[FilledCircle]", 4}, {"-", 12}}, PlotStyle -> {Blue, Green}, PlotLegends -> LineLegend[{None, "\!\(\*SubscriptBox[\(QBER\), \(UB\)]\), Eq. (8)"}, Joined -> True]];