In order to enter a code, it is required that you collect all Enigma Code Collectibles in the game that are necessary for that code. You can find a collectibles guide
HERE.
Code 1:01 02 03 04 05 06 07 08 09
08 07 06 05 04 03 02 01 09
The earliest this can be unlocked is after Chapter 4. This will also unlock "999 Mode".