AttributeError: module 'tensorflow' has no attribute 'placeholder'

This error occurs when you're trying to use the `placeholder` function in TensorFlow, but it's not available in the version of TensorFlow you're using.

In TensorFlow 2.0 and later versions, the `placeholder` function has been removed and replaced with the `tf.keras.Input` function. So, instead of using `placeholder`, you can use `tf.keras.Input` to create an input tensor.

Here's an example of how you can replace `placeholder` with `tf.keras.Input`:

import tensorflow as tf

# Define your input shape
input_shape = (None, 28, 28, 1)

# Create an input tensor using tf.keras.Input
inputs = tf.keras.Input(shape=input_shape)

# Continue with the rest of your model
...

If you're using an older version of TensorFlow and need to use `placeholder`, you can try downgrading your TensorFlow version or installing an older version of TensorFlow that still supports `placeholder`.