In jupyter/Beyond_Linear_Programming.ipynb, line 6 of cell [5] should be in cell [7], which is wrong.
Then, between cell [4] and [5], wrong lines have been pasted from the previous notebook (desk and cell phones).
Then, to respect the graph, in a cell [5] these two lines should be added:
for node in [(1, 4), (2, 3)]:
tm.add_constraint(x[node] == 0)
or something similar to explicitly avoid those arcs.