I got stuck at this.
callback was already in the assignment but seems like mistake is in callback.
train_generator I did data augmentation as in the week 2.
Please see attachment for the error log.
Thank You
DP
Key lookup failure on ‘accuracy’ causes the left hand side of the inequality to be None. Make sure that line and the metrics definition use the same key. Old versions of TF accepted ‘acc’ and ‘accuracy’ as equivalent but new versions require exact match. Check for that first. Let us know?
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oops my mistake 
and I knew this and I still ignored. I am sorry
Thank you for the answer. I was so confused as to why the callback section was failing and without the callback the model was able to run. Now I know so thanks again.
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