It's LUTXV for the Plat (and Reserve if used for main cabin). For the Reserve card, It's the two lowest discount F classes (I and Z). I would first check if there is LUTXV on the flights you are looking at and, if not, see if there are other dates with availability if you have flexibility. If you find dates with LUTXV availability and the flights don't show when you try to use the cert, you might want to give Delta a call.