Re: your error
BSplinePlot2[pts : {{_, _} ..}, knots_, opts : OptionsPattern[{Plot, BSplinePlot}]] :=
Module[{p = Length@First@Split[knots] - 1, a, b},
{a, b} = {First[knots], Last[knots]};
With[{a1 = a, b1 = b},
ParametricPlot[
Total@MapIndexed[NBSpline[First@#2 - 1, p, knots, u] #1 &, pts], {u, a1, b1},
Evaluate[Sequence @@ FilterRules[{opts}, Options[Plot]]],
Epilog -> If[OptionValue[ShowPoints],
Join[Text @@@ (Thread@{Table[ Style[Subscript["P", i], 14], {i, 0, Length@pts - 1}],
# + {.4, .2} & /@ pts}), {Red,PointSize[0.015],Point[pts], Green, Line[pts]}]]]]]]]]]
pts = {{0, 0}, {0, 2}, {2, 3}, {4, 0}, {6, 3}, {8, 2}, {8, 0}};
knots = {0, 0, 0, 1/5, 2/5, 3/5, 4/5, 1, 1, 1};
BSplinePlot2[pts, knots, PlotRange -> {{0, 10}, {0, 5}}, ShowPoints -> True]
