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Which store did you get the game from? The itch.io version should come pre-patched, with "TNLT_adult_patch.rpa" already in the game folder.

Also, which platform? Windows, Mac, Linux, Android?

windows steam

Do you have the option of installing the 18+ DLC from Steam directly? If it's available in your region, that would be the easiest option. Steam should automatically put the file in the right place.

If not, it should be a matter of finding the game in your steam library, right clicking on it and going into Manage -> Browse local files. Then open the "game" folder and copy the .rpa file into that folder. The game needs to be restarted after that (if it's currently running).
Also, make sure you downloaded the patch from https://zetsubou.games/tomboys-need-love-too/index.html
If you downloaded it from somewhere else, it's possible that it's an old version of the patch made for a previous version of the game.

i downloaded it from there though the patch still doesnt work

Can you take a screenshot of the patch file inside of the game folder, so I can see if you've got the right file in the right place?

I'm having the same problem. I have the patch in the game folder and yet in the game, it still says it's not installed. did anyone ever figure this out?

Can you follow the instructions in the post and upload a screenshot of the file where you've placed it so I can check if it's in the right place?