Yes, as Kin says, you must have modified the comments in at least one of the function cells. Here’s what the first few lines of the first graded cell should look like:
# UNQ_C1(UNIQUE CELL IDENTIFIER, DO NOT EDIT)
# GRADED FUNCTION: triplet_loss
def triplet_loss(y_true, y_pred, alpha = 0.2):
If you removed those comment lines, then the grader will fail. Note that they even warned you not to mess with that line. 
It is generally a mistake to modify things outside the “YOUR CODE HERE” sections. It’s not against the rules to add code outside those areas, but you need to be very careful that you know what you’re doing when you do that.
If you need more details about how to get a fresh copy of the notebook, here’s a thread about that.