Are you from Germany?
Steam blocks the game page due to your country's laws against adult content.
If you want to buy the Steam version, you can support us on Patreon and we will generate a key for you. With the key, you will be able to access the game.
https://www.patreon.com/posts/living-in-itch-75512700
If you don't care about the Steam version, you can buy it directly from the OppaiMan Store.