The notebook has several different ways of defining the validation indices. If you just run the notebook cells in order, you set val_idx=[0] (what you do before training with the entire dataset) before forming your model. If you ran the model with a single validation index, you would be comparing your rmspe to a single value. See what your validation set is, and if it is a single value, try change it to the last two weeks of data via:
val_idx = np.flatnonzero(
(df.index<=datetime.datetime(2014,9,17)) & (df.index>=datetime.datetime(2014,8,1)))