Thanks to Varun - I was able to proceed with the installation.
However, I am stuck with the following error during replication server installation on primary server.
"Unable to connect to standby database as user DR_admin. The standby environment needs to be set up first before replication configuration can be done"
When I try installing RS on DR server, I get the same error for setting up primary environment 
Have I missed anything here? I did select the option to configure replication server in SWPM as per below
If I understand correctly, the DR agent configuration needs to be done once replication server is installed on primary and DR servers. Please correct me if I am wrong.
Any pointers will be appreciated.
Thanks again
Regards,
AJ