The error ValueError: Only instances of keras.Layer occurs when you add an invalid object (like a string or uninitialized layer) to a Keras model.

Common Causes: Non-Layer Object: Adding something like a string or function instead of a Keras layer.

Fix: Ensure all objects in the model are valid Keras layers. python Copy code model = Sequential([Dense(64, activation='relu')]) # Correct Uninitialized Layer: Forgetting to initialize a layer.

Fix: Add parentheses when defining layers. python Copy code layer = Dense(64) # Correct

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