It's expected your answer to equal expected answer, so the expected value in the error message is 1 not -1. -1 is the value your function returned. Not a kata issue.

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

Add this print(g, m, n) as the first line in step function and you'll see the input. Check this out: https://docs.codewars.com/training/troubleshooting#print-input

The problem is in your code, it ignores the boundaries and returns numbers outside the range [m, n].

Language, values, Sample tests or Attempt tests?

It's not a problem in the tests, try calling your function locally like in the tests, that is, one after the other:

print(longest_slide_down([[3], [7, 4], [2, 4, 6], [8, 5, 9, 3]]))
print(longest_slide_down([
            [75],
            [95, 64],
            [17, 47, 82],
            [18, 35, 87, 10],
            [20,  4, 82, 47, 65],
            [19,  1, 23, 75,  3, 34],
            [88,  2, 77, 73,  7, 63, 67],
            [99, 65,  4, 28,  6, 16, 70, 92],
            [41, 41, 26, 56, 83, 40, 80, 70, 33],
            [41, 48, 72, 33, 47, 32, 37, 16, 94, 29],
            [53, 71, 44, 65, 25, 43, 91, 52, 97, 51, 14],
            [70, 11, 33, 28, 77, 73, 17, 78, 39, 68, 17, 57],
            [91, 71, 52, 38, 17, 14, 91, 43, 58, 50, 27, 29, 48],
            [63, 66,  4, 68, 89, 53, 67, 30, 73, 16, 69, 87, 40, 31],
            [ 4, 62, 98, 27, 23,  9, 70, 98, 73, 93, 38, 53, 60,  4, 23],
            ]))

And see what you get.

Have you tried locally calling your function more than once? The problem is, memo isn't reset, it keeps its value. Also point being a concatenation of x + y, how would you distinguish between '11' + '10' and '111' + '0'?

Sure, but read this first: https://docs.codewars.com/training/troubleshooting#post-discourse

And what's the code that does that?

First thing first, don't change the function's name, the tests will look for count_by.

The test is fine, read the description again.

Node v18 is enabled now. Closing.

No:

Assume the key is repeated for the length of the text, character by character. Note that some implementations repeat the key over characters only if they are part of the alphabet -- this is not the case here.

It's been already discussed 4 years ago: https://www.codewars.com/kata/52d1bd3694d26f8d6e0000d3/discuss#6050b777d2342a000e60697f

Because:

Also if you have trouble discerning if the particular sequence of 1's is a dot or a dash, assume it's a dot.

Did you read this yet? https://docs.codewars.com/training/troubleshooting