Help ! Why am I repeatedly getting a zero even if the code is doing what it is supposed to do

Pls do check!!
The callback works and the code runs great but its still graded zero.

Can you see what myCallback is writing, it says a local variable ‘training_images’ referenced before assignment. This means that this variable where is used (and you need to find the place where its used) is called or used before any value on it has been assigned.

Thanks a lot.
I had not used any training_images and that is why i was totally confused as to why it was asking me for that variable. Maybe because i had run the example program before in colab. No idea, i assume it was the case
So what i did was cut the code and pasted it in a new notebook where it worked perfectly and i got scored. Thanks so much