If you need to capture the an installer for the original version of macOS that came with the computer you can do this via the Recovery Partition.
- Boot to the recovery partition , hold Command and R on boot or Internet Recovery hold Alt, Command and R
- Use Disk Utility to wipe the Macintosh HD volume only
- Select Re-install Mac OS
- This will download the installer
- Very important, after the initial download it will reboot the Mac. At this point we need to turn it off. Do not let it boot up.
- Boot the Mac into Target mode, Hold T as you turn it on. Attach this Mac to another computer using a thunderbolt / Firewire cable
- Its HD will mount on the other Mac. On there will be a disk image called "InstallESD.dmg. This should be in a directory called "Mac OS X Install Data Files". We need to copy that to the Server