If you get this message for a game on Steam it is not available in your region.
It happens a lot lately in my country Germany, because every game that hasn't an age rating is simply blocked.
To get info for such a game you can use https://steamdb.info/ and just search for the appid on the webpage or simply add it to the url like this:
Example: Abyss Fantasia with the appid 1985940.
If you find such a game you can send a mail to the developer and mention that an age rating is mandatory for Germany since November 15. Link: https://steamcommunity.com/groups/steamworks/announcements/detail/4678768276768588864
Every developer can rate his game by himself, but they have to do it.
It happens a lot lately in my country Germany, because every game that hasn't an age rating is simply blocked.
To get info for such a game you can use https://steamdb.info/ and just search for the appid on the webpage or simply add it to the url like this:
Example: Abyss Fantasia with the appid 1985940.
If you find such a game you can send a mail to the developer and mention that an age rating is mandatory for Germany since November 15. Link: https://steamcommunity.com/groups/steamworks/announcements/detail/4678768276768588864
Every developer can rate his game by himself, but they have to do it.