What’s the model number of the LIFX bulbs? Are they on the list of supported devices that appears in the app? While nothing is guaranteed, that list is usually good.
If they aren’t on that list, contact LIFX support and ask them. (Since SmartThings published their API about 18 months ago, device integrations, with a very few exceptions, are now the responsibility of the device manufacturer to create and maintain.)
The LIFX integration is used by quite a few community members. According to the LIFX website it is a cloud to cloud integration, so you need to first set up the lights with an LIFX account and their app and make sure everything is working that way.
https://support.lifx.com/categories/smartthings-H1G9XBrt
Once that’s done, open your smartthings app and add the device to SmartThings from there.