Assalamu Alaikum, If I am praying behind someone and I make a mistake (such as forget to say Rabbana wa lakal Hamd) that requires Sujud As Sahw, should I do the two prostrations after the Imam finishes?

Ustadh AbdulAziz al-Haqqan

No, there is nothing upon you and the imam carries it, unless you were late for the salah and you have to make up 1 or more rakah then you do.