Apologies if this has been posted before, but I didn't see it anywhere.
I did a Patrol mission in the Regulus sector block (I forget which system it was) which gave me 10 diplomacy XP and pushed me onto Tier 1 of the Diplomacy branch. Then a window popped up on my screen. It was a communication from a Caitian offering me a green Duty Officer from the Klingon faction. I accidently closed the window among other windows I had open and couldn't find a way to open it again, so I left it, thinking I would probably get another chance to get my DOff when I hit Tier 2 in Diplomacy.
Last night, I hit Tier 2, and got my blue DOff, but while it showed me as still having the token to trade in for my green DOff, it wouldn't let me purchase one.
This is a known Doff bug... The work-around (currently) is to remove all of your Doff from your active roster (space/ground) as purchased Doff are placed there (the bug) and if you have all 5 active slot allocated... You can place your Doff back in their slots after the purchase...