You created a WinPE-based bootable media.
When you boot a machine with Windows 8 or later, or Windows Server 2012 or later with this bootable media, Disk Management option is missing.
This is the expected behavior.
WinPE-based media supports Disk Management utility for Windows 7 and below. Disk management for Windows 8 and 10, and Windows Server 2012 or later is not supported in WinPE of Acronis Cyber Backup 12.5 and Acronis Cyber Protect 15.