Learn String Manipulation by Building a Cipher - Step 16

You’re not the only one.

Here are a few topics where this has been explained, maybe one will work for you:

https://forum.freecodecamp.org/t/learn-string-manipulation-by-building-a-cipher-step-16/656960

https://forum.freecodecamp.org/t/learn-string-manipulation-by-building-a-cipher-step-16/658331

https://forum.freecodecamp.org/t/learn-string-manipulation-by-building-a-cipher-step-16/658121