https://www.gamespot.com/articles/destiny-2-mida-multi-tool-exotic-quest-bug-found-h/1100-6453210/

Destiny 2 developer Bungie has confirmed it's aware of another issue with the game and is currently on the case to fix it. The official Bungie Help Twitter account today acknowledged a problem where the MIDA Mini-Tool does not get sent to the Postmaster after completing the "Enhance" quest.

Until a fix is made, you can prevent this issue from happening by making sure you have more than one Energy weapon inventory slots available when you complete the quest, Bungie said on its support site. Presumably a permanent fix is coming later.