We are currently trying to resolve this issue. Apparently, users outside of the US have been unable to download DwarfCorp from Steam. We apologize for the confusion and are working on the issue with Valve.
In the meantime, you can purchase DwarfCorp at our itch.io page. From there, you can get a Steam key which will work for you:
https://completelyfairgames.itch.io/dwarfcorp