This cryptic achievement literally has the solution in the name. It says 20X and the Redeem Bonus Code in the Shop shows the description for what a code should normally look like which has twenty X's
To unlock this achievement head over to the shop in-game. Click on redeem bonus code and then copy and paste or type out in all caps in the box. Once done hit enter for an icon and achievement.
XXXX-XXXX-XXXX-XXXX-XXXX