Portal will not start new game when I click on "Start New Game"

I just got Portal and I can't seem to start a new game. Initially, clicking on "New Game" on the main menu showed a dialog box with no content.

I've tried to find the answer online and saw to copy the files from the portal folder to the hl2 folder (C:\Program Files (x86)\Steam\steamapps\common\Portal has both of these folders within it). This now shows a list of "chapters" when I click on "New Game", but nothing happens when I click on "Start new game". I've tried clicking on Chapter 1 and then starting, I've tried uninstalling and reinstalling Portal, I've also uninstalled and reinstalled Steam but to no avail.

The dialog box closes when I click "Start new game" and then nothing else happens; I'm still on the main menu. Everyone else who I've found online who has this problem solved it by copying the files to the hl2 folder or reinstalling the game, but neither of these solutions have completely solved the issue for me. I have Portal 2 installed as well, and it works perfectly fine.

Does anyone know how to fix this or a situation like mine that has been solved?