Earlier, PS Store kept telling me that I could not buy Forsaken expansion because I did not own the Osiris and Warmind expansion, which I actually did. So what I did was to "buy" the Curse of Osiris and Warmind expansions in PS Store. When I clicked on the Buy option, it led me to a Download option. So I "downloaded" again. In actual fact, I did not pay for the Buy, and the "download" did not physically happen. But by doing these actions, PS Store acknowledged me an the owner of the Osiris and Warmind expansions and I was able to buy the Forsaken expansion :-)