Sorry, but if you bought the game from itch.io, your game version will read XXXX.XX.XXi(with the "i" representing that you bought it from itch.io), and because itch.io does not allow in-game DLC, only versions with the types XXXX.XX.XXs, XXXX.XX.XXo, XXXX.XX.XXa and XXXX.XX.XX[for switch users only] can get DLC(with the switch version being an exception currently). If you ask me, the Xs represent the numbers of your version, and the letter after the numbers represent where the game was paid from/what platform it's being used it, and if there is no letter after the numbers in the version of the game, you are using a switch version. The itch.io version also isn't updated very frequently(same with the mobile ports).