Make sure to read the FAQ, as it explains why you are unable to purchase the DLC. Also, if possible, please visit the"14 Nights With You" Itch page and provide a screenshot of how you've installed the DLC so I can try to help you further. As stated above, this comment section is mainly for the base game, and any DLC concerns should be directed to the DLC page.
Viewing post in 14 Days With You [DEMO] comments
If the scenes are not appearing even after enabling the correct settings, then it means there is something wrong with the DLC. Choosing the 18+ option at the start does not impact the DLC in any way.
Again, as Vin has said, if you need further help with troubleshooting, please leave a comment on the DLC's page, and I can try to assist you further.