When your Quest / Bounty Inventory is full when completing the first Mission in Shadowkeep, the Unidentified Essence (which gets dropped) goes into the Postmaster, but when you talk to Eris, before picking up the Unidentified Essence from the Postmaster, you later can't give Eris the Unidentified Essence as stated in the notes of the Essence. So you basically have a quest item you can't get rid in your Inventory (as seen in linked Image)