In order to purchase the stand alone TTK DLC you are required to own all previous content on your account already. The other alternative would be to purchase the legendary edition of TTK or the collectors edition of TTK which includes all previous content. You can find more information and available options here https://www.bungie.net/en/Help/Troubleshoot?oid=14081 https://www.destinythegame.com/taken-king/wheretobuy