Double check the login/ password. Try quitting and relaunch Launcher then game. Try this a few times.201 - Authentication Failed
Players are receiving this message because they aren't using the correct username to log in or because there are general connectivity issues between a player and the ESO service. If you are using the correct user name, please ensure that you either have your anti-virus or Firewall set to allow all possible instances of ESO, or disable it completely.
Alternatively, if you were able to log into the game before and start seeing this error, please wait a few minutes and try again as it may be a temporary hiccup.