This game requires files from Portal 2, so a reminder is that you must both OWN and HAVE INSTALLED PORTAL 2!!! I have 2 solutions to the problem of the game crashing, they work for Mac and Windows.

This is the most common problem. You must have PORTAL 2 AND APERTURE TAG installed ON THE SAME DISK! If you do not own Portal 2, i'm sorry, you can't play, and if you do, you must instal it. If it is installed but it still does not work, make sure it is on the same hard drive, if they are, go to solution #2

If you have Portal 2 installed and Aperture Tag installed on the same disk, and it still crashes, then Portal 2 updated and they need to be compatable. To fix this you need to find the Steam folder which is in Program Files/steam and locate the Portal 2 and Aperture Tag files which should be in Program Files/steam/steamapps/common. For a mac these will be in Application Support/Steam/steamapps/common. Open the Portal 2 folder and copy the bin folder, it should be the first one. Go back to common and into the Aperture Tag folder. Paste the bin folder from Portal 2 into Aperture tag. If there is already a bin folder replace it. Now do the same but go to Portal 2/portal2 and take that bin folder, and paste it in Aperture Tag/aperturetag, if there is already a bin folder replace those.

REMEMBER! First check if they are on the same hard drive and if Portal 2 is installed then try copying the bin files. Thanks for reading, feel free to ask questions, and make sure to rate so others can see!