Error about "nonetype"

Hello, why did I make this mistake?

[code removed - moderator]

0/34Score: 0 of 34
Grader output
There was a problem grading your submission. Details: ‘NoneType’ object is not iterable

Please see this link to learn about grader metadata.
If grader metadata is missing, refresh your workspace and try again.
See Refresh your Lab Workspace section here
If you’ve already done this, contact coursera help

Should you be unsure if this is a metadata issue or your solution is wrong, click my name and message your notebook as an attachment.

Please change the metric to accuracy from acc throughout the notebook. Rest of your code is fine.

thank you, I passed!