After completing action phase, both the debt and coffers popups come up, but no matter what I do I get no opportunity to play the treasures in my hand.
Game #155920281
Final decision point
Not sure which server
You were in a Voyage turn and had already played 3 cards from your hand. So you are not allowed to play any more cards from your hand, including Treasure cards, as per Voyages' text:
Quote+1 Action
Take an extra turn after this one (but not a 3rd turn in a row), during which you can only play 3 cards from your hand.
(emphasis mine)