Hello @Burak_Bakkar
Seems like either you are using an obsolete copy to do the assignment. Your grader cells do not match with my updated assignment grader cell. or either you edited the grader cell. By any chance only way around to do the correction, is to get a fresh copy of the assignment and re-do the assignment.
Also make sure write codes only where it is asked to do so, per se Write codes were None is mentioned between ###START AND END CODE HERE####.
Also for calculation loss function, please refer the below link to write the codes correctly, the way you have recalled sparsecategoricalcrossentropy is incorrect.
How to get a fresh copy? refer the below link and follow the steps, make sure you save a copy before.
Regards
DP