Counter-Strike Global Offensive No Steam account token was specified.

From Serenity Servers Support

Well, here's the story. A few months ago they decided it was time to move to the same system as TF2 servers, mainly to keep users from altering the Skins and what not. But in any case, lets show you how to fix it!

Notice of changes to CS:GO Game Server Operation CS:GO dedicated servers will soon require Steam Game Server Login Tokens (GSLTs). This is the existing system that TF2 uses for Favorites migration.
This transition will happen in three stages:
§ Stage 1: The GSLT creation utility goes live: Please test the interface and report back any issues in creating accounts. Note that TF2 server operators can now use this form to create GSLTs for TF2 (AppId 440).
§ Stage 2: An update to CS:GO's dedicated server will include the sv_setsteamaccount convar that allows you to log in with your GSLT. In this stage it will still be possible to run a dedicated server without a GSLT. Anonymous login will happen automatically if no game server login token is specified. Please report back any issues with logging in using the game server login token or otherwise.
§ Stage 3: Anonymous logins will be disabled in CS:GO. Please report back any issues.
  1. You MUST have a phone number set in your steam account's settings, this is the only requirement for this to work.
  2. Go to this link if you are not logged in, press login at the top.
  3. Now you must enter in the details as seen below, its pretty self explanatory but here's a screenshot.
    csgo gslt
  4. After entering in the details, a token is generated as seen here.
  5. Copy the code as seen in the red box, and enter this into your servers, server.cfg file. This can be found under Configuration Files on our gamepanel.
    sv_setsteamaccount "75F5CXXXXXX5CE2XXXXXXXXXXXXXX"
    that is how it should appear in the server.cfg
  6. Save and restart, server should now appear online.