So there's this thing called a redistributable for Visual Studio 2015 that has to be installed in order to run EC. For some reason, it hasn't installed automatically like it should. Usually this is done automatically by Steam, but for some of our users this hasn't been the case.
At the moment, one way to check that this is the problem you are experiencing is the following:
1. Go to your Steam Library.
2. Right click on the game.
3. Go to "Properties".
4. Hit the “Local files” tab.
5. Go to “Browse local files”.
6. Go in the folder: “Warhammer 40,000 - Eternal Crusade\EternalCrusade\Binaries\Win64”.
7. Locate EternalCrusadeClient.exe and double click on it.
Once you do this, you won't be able to progress much in the game. However, if you do have this specific issue, you'll get a popup complaining about missing api-ms-win-crt-runtime-l1-1-0.dll.
The solution? Install the files needed. You can find them here:
Win 10: https://www.microsoft.com/en-ca/download/details.aspx?id=48145
Win 7: https://www.microsoft.com/en-us/download/details.aspx?id=49093
Once installed, restart Eternal Crusade. You might have to restart Steam as well. Hope this helps!