If you've made your purchase within the last 7 days, please contact Itch Support to request a refund ^^
And this isn't to guilt-trip you, but it might be a while before the 18+ DLC comes to Steam (right now, only the base game has been approved for Steam, and it's still in the wishlist phase until I add more content) ;v;
So if you prefer, I can try to help troubleshoot why you can't install the DLC! Just reply with a screenshot of what the DLC looks like inside your "game" folder, and we can go from there. You can also email me if you'd like more privacy.
If not, then I suggest getting in touch with Itch Support to try and get a refund! Thank you for your support nonetheless!!