Hey @Joseph-Marc_Antoine,
Welcome and we are glad that you could be a part of our community 
Just to make sure we are on the same page, your beginning of the code cell should be like follows:
# GRADED FUNCTION: cofi_cost_func
# UNQ_C1
def cofi_cost_func(X, W, b, Y, R, lambda_):
Please copy and paste this, if it’s anything different, and make sure you don’t put in any extra spaces. If this doesn’t work, feel free to follow the instructions in this thread, to get a fresh copy of your notebook. I hope this helps.
Cheers,
Elemento