Hi @Anwesh_Kota
Sessions will not be supported by a 3.6 cluster until 3.6 features are configured to be available. Please see the documentation at https://www.mongodb.com/docs/manual/release-notes/3.6-upgrade-replica-set/#enable-backwards-incompatible-features for instructions on how to do that.
If you’ve already done that procedure and sessions still don’t work, please let us know.