السلام عليكم ورحمة الله وبركاته
What happened is that the Imaam had recited an Aayah of Sajdah. When an Aayah of Sajdah is recited, then it is obligatory upon both the reciter and the one who hears it to make Sajdah. In your case, it was obligatory upon you to have made Sajdah along with the Imaam. However, you did not know, and thus did not do it. The
Mahall (place) of an Aayah of Sajdah recited in Salaah is the Salaah itself; the moment the Salaah is over, the time to carry out that particular Sajdat-ut-Tilaawah has passed, and the one who missed it has to make Istighfaar (seek forgiveness from Allaah Ta`aalaa for having omitted it), but there is no Qadhaa for that Sajdah Tilaawah.
والله تعالى أعلم
والسلام