The problem is in calculating the cost. Multiplying two matrices element-wise is different form matrix multiplication which uses the dot product operation. If you read the implementation instruction, it suggested to use the comput_cost() function that you have already implemented in the previous lab. All you have to do is just call the function and pass in the parameters. The function is linked to the lab notebook, please see the import statement at the top:
from dnn_app_utils_v3 import *