You cant on Itch.io because of Collective Shout forced the payment processors to ban adult games from being able to use payment.
You can however buy it on DLsite's store.
Viewing post in Syahata's bad day(v1.0.5) comments
You can try this link out, but i dont know if you need an account in order to see the page.
https://www.dlsite.com/maniax/work/=/product_id/RJ01140781.html