The base game for this DLC is free and you will need to add it before you can add the DLC I believe.
Click the following link and click install in the upper right corner: https://steamdb.info/app/1576340
Alternatively put steam://install/1545820 in your browser.
Do NOT go to normal Steam page as it is not free there.
I put the normal page into main link but for info ONLY and because I am not allowed to put in the direct link.
Credit and thanks to Kaizuken.