"Site Recovery> Network Mapping" may not show the port group under data center.VMware Site Recovery Manager 8.x and 9.x
Network details may fail to populate port groups when creating reverse mapping if the user account used to log into vCenter lacks the necessary privileges to perform the site recovery administration. As a result vCenter is unable to display the relevant network information.
Refer the below docs and ensure user account has SRM Administrator or Recovery Administrator roles. Make sure to login with admin account to perform the necessary task.