issues linking with steam

[BootToTheScrotum]

hello im fairly new to this, ive been trying to set up a new account for my bro for a while now. i set up a steam account for him, and myself for that matter. i even purchased another copy and linked him to it even though i already had the game. however even after linking his account, making sure his account was public first, when he tries to log in it says he has to authentify his steam account. im at a loss as to what to do. can you provide any insight?