The E_INVALID_STEAM_TICKET error is indicated when something happens in the background while Steam is trying to connect your game to our servers.
Most likely there was something like the game accidentally being closed, or a temporary loss of internet connection that may have affected server connection. Try restarting your computer, ensure that you are on a wired (not wireless) internet connection, and try creating an account once more.
(Note: Do not close or exit the game when trying to create an account – Don't Starve Together must be running in order for Steam and the game to connect to our servers)
As well, you may want to check Valve/Steam's support site with troubleshooting for this type of issue: https://support.steampowered.com/kb_article.php?ref=1817-RTSK-8917