General Issues:

* Check that Steam is running

* Check that Steam is in online mode

* The server has reserved slots and may be full

* You may of been banned by the server administrator

Next:

* Verify the integrity of your game files (Steps in #user-faq 6)

* Run the game through Steam and not it’s executable

* Restart Steam

* Restart your PC

If the issue is still not resolved:

* Go to Program Files/Steam/steamapps/common/SCP Secret Laboratory

* Right-click SCPSL.exe

* Click Properties

* Go to the Compatibility tab

* Check "Run as Administrator"

* Hit Change for all users

* Start the game via your Steam Library