itch.io

if you bought the dlc first and didnt reinstall the game it wouldn't activate. I'm unsure as to if downloading the files from itch.io first would also cause issues - but either way you need to reinstall it again after purchasing the dlc. I'm unsure as to if the dlc has ever been free on steam - are you sure you didn't just buy the base game?

did you buy the dlc that cost money first? also, if you got the original files from here that may cause issues