ValueError: ...incompatible with the layer: expected axis -1 of input shape to have value 20 but received input with shape (None, 20, 637)

I hit with the error message on my model; 'ValueError: Input 0 of layer sequential_5 is incompatible with the layer: expected axis -1 of input shape to have value 20 but received input with shape (None, 20, 637)'

Im not sure how to fix this. Code as below;

print(str(audio_train.shape)+''+str(y_train.shape)+''+str(audio_valid.shape))

(700, 20, 637) (700, 2) (236, 20, 637)

model=Sequential()
###first layer
model.add(Dense(100,input_shape=(20,)))
model.add(Activation('relu'))
model.add(Dropout(0.3))
###second layer
model.add(Dense(200))
model.add(Activation('relu'))
model.add(Dropout(0.3))
###third layer
model.add(Dense(100))
model.add(Activation('relu'))
model.add(Dropout(0.3))
###final layer
model.add(Dense(2))
model.add(Activation('softmax'))
model.summary()

adam = tf.keras.optimizers.Adam(learning_rate=0.0001)
model.compile(optimizer = adam, loss = 'categorical_crossentropy', metrics = ['accuracy'])
#fitting the model
Au_model = model.fit(audio_train, y_train, batch_size = 32, epochs = 10, validation_data = (audio_valid, y_valid), verbose=1)

Error message as below;

ValueError: in user code:

C:\Users\seren\anaconda3\lib\site-packages\tensorflow\python\keras\engine\training.py:855 train_function  *
    return step_function(self, iterator)
C:\Users\seren\anaconda3\lib\site-packages\tensorflow\python\keras\engine\training.py:845 step_function  **
    outputs = model.distribute_strategy.run(run_step, args=(data,))
C:\Users\seren\anaconda3\lib\site-packages\tensorflow\python\distribute\distribute_lib.py:1285 run
    return self._extended.call_for_each_replica(fn, args=args, kwargs=kwargs)
C:\Users\seren\anaconda3\lib\site-packages\tensorflow\python\distribute\distribute_lib.py:2833 call_for_each_replica
    return self._call_for_each_replica(fn, args, kwargs)
C:\Users\seren\anaconda3\lib\site-packages\tensorflow\python\distribute\distribute_lib.py:3608 _call_for_each_replica
    return fn(*args, **kwargs)
C:\Users\seren\anaconda3\lib\site-packages\tensorflow\python\keras\engine\training.py:838 run_step  **
    outputs = model.train_step(data)
C:\Users\seren\anaconda3\lib\site-packages\tensorflow\python\keras\engine\training.py:795 train_step
    y_pred = self(x, training=True)
C:\Users\seren\anaconda3\lib\site-packages\tensorflow\python\keras\engine\base_layer.py:1013 __call__
    input_spec.assert_input_compatibility(self.input_spec, inputs, self.name)
C:\Users\seren\anaconda3\lib\site-packages\tensorflow\python\keras\engine\input_spec.py:251 assert_input_compatibility
    raise ValueError(

ValueError: Input 0 of layer sequential_5 is incompatible with the layer: expected axis -1 of input shape to have value 20 but received input with shape (None, 20, 637)

Appreciate your advice/suggestion.