Assassin's Creed Black Flag Crash fix.

The latest Steam client beta build is preventing some games from starting, if you're using it switch back to the stable build.

Steam > Settings > Interface > Client Beta Participation > No beta chosen

1. Find the file: Locate the "AC4BFSP" file
2. Right-click on the file: Select Properties from the context menu.
3. Go to the "Compatibility" tab: In the properties window, find the "Compatibility" tab.
4. Select compatibility mode: Check the box that says Run this program in compatibility mode for, then choose Windows XP (Service Pack 3) from the list.
5. Apply the changes: Click Apply, then OK.

Now, the "AC4BFSP" file should run in compatibility mode with Windows XP (SP3).

It also helped me to launch the game through the exe file In the main game folder. "AC4BFSP"