I'm guessing, because each encoding swaps one character with another distinct character in a loop that includes all characters, by encoding 66 times you cycle through all characters in the loop and return to the original character. Doing it 65 times stops one short of that and gives the same result as -1 time.

Are you getting some sort of error or just time out?

It is a question, not an issue of the kata but a problem of your code. You could read https://github.com/codewars/codewars.com/wiki/Troubleshooting-your-solution.