A: Some German production was printed with CD-Keys that don't properly work on a non-updated version of the game. The solution is to make sure to 'Check for Updates' during the install process and run the Update before launching the game. Once updated (to version 5.4.2 or later), the game will accept your key and start normally.
If the game is already installed, you can update to the latest version using the Check for Updates selection in the Supreme Ruler 2020 Windows Program Menu. This will automatically obtain the correct update for the version you are running. Alternately, you can download the latest update exe from BattleGoat's Update link. Once updated, run the game and enter your key, and it should work properly.