Well, I'm not underage or anything, and I even tried to buy the DLC a few times, but it always gives an error. Anyway, that wasn't the point. The thing is, when I downloaded the game, I enabled the +18 options and everything that appeared on the initial screen, but as the game progresses, none of those +18 options appear like they did before during the story.
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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.
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.