On page 910 in the code example you see the following code example
sentence1 = 'follow the white rabbit neo'
sentence2 = 'no one can be told what the matrix is'
tokenizer(sentence1, sentence2)
Later the variable joined_sentences is used, but the assignment is missing.
The correct version is in the corresponding jupyter notebook:
sentence1 = 'follow the white rabbit neo'
sentence2 = 'no one can be told what the matrix is'
joined_sentences = tokenizer(sentence1, sentence2)
joined_sentences
On page 910 in the code example you see the following code example
Later the variable joined_sentences is used, but the assignment is missing.
The correct version is in the corresponding jupyter notebook: