Reconfirming the browser in Steam may resolve some "Unable to Connect" error messages in-game.
- Open your web browser (Microsoft Edge, Chrome, Firefox, etc.)
- Go to the Steam website
- Click the login button near the top right of the screen
- You will be prompted to enter a code that will have just been sent to your email address (the one that's registered with your Steam account)
- Copy and paste the code from the email