Obtain a Treasure Demon Achievement?

[Siamese55]

I have a Regent and Queen's Necklace, but this still hasn't popped. Is it a specific treasure demon?

[RezetteBombe]

That happened to me too. Some say it unlocks only if you obtain the treasure demon without remove one of your current personaes cause you don't have enough slots. In my case it unlocked randomly after I obtained a treasure demon in Mementos

[Louis Cypher]

If you mean the guaranteed scene at the 2nd palace and caught the demon without having a free slot, the condition won't trigger.
Don't worry though, just get one in mementos while having a free slot.
In mementos they appear not as shadows but red crystals, so you can free a slot before approaching and get the achievement afterwards.

[Siamese55]

It was the free slot. After the guaranteed scene (I figured the scripted ones didn't count after the delay on the attack achievements) I captured a Regent and Queen's Necklace and was starting to get concerned.

But after releasing a persona and grabbing one in Mementos it popped.