It means you found a typo.

Fixed. Thanks.

Locally you're not using the same input from the tests, not a kata issue.

lines = ["CAT"]
rotors = [[1,13,27]]
result = ["DOG"]

In your code, with that input value before + rotors[i] makes no sense, before is a number and rotors[i] is either an array or undefined depending on i value.

rotors // array of array-of-rotor-values. Each array-of-rotor-values is applied to the corresponding display line