Hello milo08, Google Play does not allow NSFW content. This is why the game is less expensive there than on itchio or Steam. We are not allowed to even SAY it's censored either, nor to link to the NSFW version, or else Google Play will ban the app (it happened before). They are even more restritive than Nintendo and Playstation.
To answer your question: I'm sorry but you can't redeem an itchio copy with a Google Play purchase.
You're welcome to return your Google Play copy and buy it here though.