That is a very generic error that something is not set up properly in your game.
1 - If you've installed any plugins lately, make sure you're starting a new game, not loading a save.
2 - If you've installed any plugins at all, make sure they're all in the correct order according to the author's instructions.
3 - Check that your party members in the System tab of your database are existing Actors defined in that tab.
4 - Check all of your enemy Troops that could be loading and make sure they refer to existing Enemies defined in that tab, nothing you've changed or deleted.
If all of that checks out, then when you get that screen, hit F8, go to the console, and post a screenshot of it here.