Retargeted animations to FN_Mannequin do not appear in sequenced character device. You need to retarget to SK_Male_Base_Skeleton for them to show and I feel it is hard for a user to understand this and ALSO you don’t see a preview if you do that, so you don’t know if the retarget was correct.
It would be nice if the animations show if you retarget to FN_Mannequin instead, OR if the SK_Male_Base has a mesh that helps us see the retarget animation limbs correctly (and ideally a setting for each Body type in Fortnite so we can get limbs not overlapping)