(n,) and (n,1) are not the same shape. Try casting the vector to an array by using the [:, None]
notation:
n_lists = np.append(n_list_converted, n_last[:, None], axis=1)
Alternatively, when extracting n_last
you can use
n_last = n_list_converted[:, -1:]
to get a (20, 1)
array.