I've purchased Dunegon Town, and I notice in the store page there's a free DLC called Dungeon Town UNRATED. I tried to add that to my cart but it wouldn't work. and clicking the link to the DLC goes to a generic page as the DLC isn't there.

Does anyone has the same issue?

Well maybe it's region locked?
What does it do when you click this link?
https://www.gog.com/game/dungeon_town_unrated

Also i bought the game first and then i decided to get the dlc after the time i bought the game.
But if it doesn't want to work maybe you can visit kagura games site and get the patch there maybe?
They do have a patch there meant for people outside of Japan.

When I click on the link it goes to a generic game list page https://www.gog.com/en/games

so it's region locked?

Yes i believe it's region locked then, but you can still try going to kagura games site to get the unrated patch there.
The patch on kagura games site is meant for people outside of Japan.

Thanks! I just visit Kagura's website and obtain the patch there.