SimpleCipher | NodeJS implementation of Simple Cipher technique | Encryption library

I should write a encoding and decoding program in a class and then use it in main. The program needs the position for each letter to increase by 2.

When I run the program, the problem is that when I enter a string (like cookie), only the last letter is encoding. Here is a Screenshot of program running.

What is the problem for my program.

Thanks.

The lesson is very basic tho and the assignment are forbid students import any other java method like base64.Only use the starter code.

The code I will put here as well

public class SimpleCipher {