Merging within the Provisioning Service console is only possible for an individual vDisk version.
The vDisk was NOT created with the latest version of the Provisioning Services target device software.
The administrator needs to promote the existing Maintenance Version to Test or Production, after that the Merge option becomes available.
The last production version of the vDisk is locked and used by the Target Devices.
After all locks are removed, the Merge option becomes available.