This achievement will unlock at the end of chapter two of season three.
*** Spoiler - click to reveal ***After returning from the supply run, you'll have an argument between Kenny and Lilly to go through, once this ends and Lilly tells everyone to get out, the achievement will unlock.
I've noticed a delay in the achievement unlock time on this game and other Telltale products, so don't panic if this achievement doesn't unlock straight away. The best fix for this is simply to continue playing the game, the achievements will unlock soon after.