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