Hello, and thank you for your report. For an account to be eligible to purchase a copy of Destiny 2: Forsaken, the account must own all previous Destiny 2 content, including the base game, Curse of Osiris, and Warmind. If your account does own all previous Destiny 2 content, you may need to restore your licenses. This refreshes your game licenses and can help resolve issues where the console may not be recognizing a particular game license that you own. Here is a guide on how to do so: https://www.bungie.net/en/Help/Article/12181