Exit out of Steam, and then in Windows Explorer, browse to the '..\SteamApps' folder where you are trying to install the game (to check that, in the client click the Steam menu, select Settings, and then in the Downloads section, click the Steam Library Folder button).
After that, delete or rename the appmanifest_243950.acf file, and you should be able to restart Steam and install the game. If you have Windows Explorer in details view (right click an open area of the right pane), sorting by date may make it easier to find, or if the majority of your installed library are older titles, it would be near the bottom when sorted by name.
If that doesn't help (it should), try changing your Download Region or speed, or
Refresh Your Steam Files and reboot your computer.
