Heroes in Heroes of Might and Magic V.

You can try to fix everything with a simple method. Delete all maps from the maps folder and all mods, and then add them one by one, checking if this bug appears. Or, if the previous method doesn't help, simply reinstall Heroes. In the worst case, this will take about an hour, but you'll solve the problem. I don't remember many scenarios where you can't buy heroes (and I've played quite a few scenarios).