Steam was unable to sync your files it is a common error that people get when they try to launch some of their games. Here is a simple fix:
Step 1: go to steam library
Step 2: click on 'OUT OF SYNC' next to the play button
Step 3: if step 1 and 2 doesn't work, then right click on the game you want to fix, and press 'PROPERTIES'
Step 4: select 'LOCAL FILES'
Step 5: click on 'VERIFY INTEGRITY OF GAME FILES'
