A couple of errors:

1. You are instantiating a model model = load_vgg_model("imagenet-vgg-verydeep-19.mat") then reseting the default graph tf.reset_default_graph() → it should be the other way around.
2. You have one tf.InteractiveSession() started and you are creating another tf.Session() → You don't need the first one.

This should work for you:

import os import sys import scipy.io import scipy.misc import matplotlib.pyplot as plt from matplotlib.pyplot import imshow from PIL import Image from nst_utils import * import numpy as np import tensorflow as tf

tf.reset_default_graph()
model = load_vgg_model("imagenet-vgg-verydeep-19.mat")

content_image = scipy.misc.imresize(arr = scipy.misc.imread("content.jpeg"), size=(300,400,3))
content_image = reshape_and_normalize_image(content_image)

style_image = scipy.misc.imresize(arr = scipy.misc.imread("monet.jpg"), size=(300,400,3))
style_image = reshape_and_normalize_image(style_image)

generated_image = generate_noise_image(content_image)
imshow(generated_image[0])

# Assign the content image to be the input of the VGG model.  
sess = tf.InteractiveSession()
sess.run(model['input'].assign(content_image))