Error in provide gradient

[mstfvi]

I run your codes according to your instruction,
after compiling the model this error appears:
Can you help me to fix this error?

ValueError: No gradients provided for any variable: ['embedding/embeddings:0', 'embedding_1/embeddings:0', 'gru/gru_cell/kernel:0', 'gru/gru_cell/recurrent_kernel:0', 'gru/gru_cell/bias:0', 'gru_1/gru_cell_1/kernel:0', 'gru_1/gru_cell_1/recurrent_kernel:0', 'gru_1/gru_cell_1/bias:0', 'time_distributed/kernel:0', 'time_distributed/bias:0', 'dense_1/kernel:0', 'dense_1/bias:0'].