I tried purchasing Forsaken for PS4 a several amount of times now and every time it said something on the lines of “cannot purchase”, “product expired”, “error” etc. What fixed this problem for me is to go onto the store via the PS app and check that you have selected “purchase” on both Curse of Osiris AND Warmind. For whatever reason I was able to play both dlc’s but could not purchase Forsaken as I had not authenticated Warmind but already had with Osiris. You may have downloaded and be able to play the dlcs but may not have authenticated it. If you have bought the season pass or already bought both of the expansions you can do this for free. It says “purchase” but the price total is 0£/$/€ and you don’t have to download anything. Just a mere click of the button. Hope this helps to those whom experiencing this.