The challenge is about digraphs, of course.
Roughly explained, a digraph means two symbols are resulting in the same one(or more?) letter(s).
If i recall correctly you can use JPK ascii->digraph function to convert 2 characters into 1.
JPK is not needed for any challenge here, but sometimes it can help you a bit
After you did the conversion, you are left with a simple substution cipher.
Edit2: I am not sure if my roughly explaination is 100% correct. At least you can say that you should work on the plaintext grouped in pairs. Another example of a digraph cipher may be Playfair. But not sure on that either.