In the editor, there is an AI test-runner to test balancing. Otherwise, no, there is no such mode.
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What's the error message you are getting? Something like "OpenGL 2.0 or higher with the FBO extension is required. OpenGL version: 1.1.0" in the stderr.log? If you have an Intel HD 2000/3000 graphics card, you will need to apply the following patch:
I just released Demise of Nations. Demise of Nations is a 4X turn-based grand strategy wargame covering the rise of Rome till the fall of modern civilization. Command your armies in one of the many ancient and modern countries including the Roman Empire, British Islands, Germany, Japan or the United States. From Rome to modern nations, you create your own warring experience. Wage colossal wars solo, against the AI, or take on your gaming friends in cross-platform multiplayer games. Form alliances and fight co-op style with the AI and other players for ultimate victory.
Game's Page: https://noblemaster.itch.io/demise-of-nations
Yes, it might have something to do with the itch.io app if you are able to start the game directly. However, I assume you are successfully able to start other games!?
What you could try (not sure if that's the case?) to move the itch.io app/game to a directory with only ASCII characters. E.g. from "C:\Über\..." --> "C\Uber\...". Not sure if that's where your games reside? If you have only ASCII characters for the game's path, it's odd that that error message would happen however. Maybe try to talk with the itch.io team as well?
It seems the game doesn't like that path. However, generally, you given it's the portable version it should work in any path. Do you have any special characters in the install path of the game? Can you move the path in itch.io?