Run reconciliation operations below after you restore the vCenter appliance from an image based backup.
Navigate to the vCenter Server Management Interface, https://vCenter_FQDN_or_IP:5480 , and click "Set up vCenter Server Appliance" button.
Input the root password and click [LOG IN].
Click [NEXT].
Input SSO user name and password, then click [FINISH].
Reconciliation starts. Wait 15 - 30 minutes to finish.
Reconciliation completes and you will be able to access vSphere Client.
"Failed to execute operation: Unit file is masked" when we try to start the service on VCSA 6.5
File-Based Backup and Restore of vCenter Server Appliance
VMware vCenter Server Appliance 6.7 File-Based Backup & Restore (VIDEO)
vCenter Server Appliance 6.7 File-Based Backup and Restore Walkthroughs (VMWARE BLOG)