The game is currently not available on Steam in Germany.
See SteamDB restricted_countries: https://steamdb.info/app/3567040/info/
It is probably missing an age rating.
Steam will automatically generate an age rating for the game when you fill out a questionnaire:
https://partner.steamgames.com/doc/gettingstarted/contentsurvey/germany
Sorry for the late reply.
In Germany the game either needs an age rating via the USK (PEGI is not valid in Germany) or an age rating that is issued by Valve through its own self-rating process.
The self rating process should be available for you under the following link: https://partner.steamgames.com/healthcheck/missingratingforgermany/