Here is the fix for your problem!
How to do it from here instead of following that link:
1. exit STEAM client
2. start STEAM client via 'run as Administrator'
3. select A2: OA in games list by left mouse button
4. right mouse button > menu > choose 'Properties'
5. chooce 'Local Files' tab
6. choose 'Verify Integrity File Cache'
7. wait till it's done
Also welcome to Armaholic