Are you one of the new European customers running on the new UK SmartThings servers?
If you are, you won’t be able to get LIFX working until they fix the OAuth issue that affects users on those servers. The SmartThings<–>LIFX integration is between the SmartThings cloud and the LIFX cloud and requires OAuth authentication to work (just like IFTTT).
You can read about and track the status of this problem here: