Mac and NetBoot servers are in different subnets.
The NetBoot image option does not appear as a boot option on the target Mac or Task Sequence step “Apply OS X Image” fails with the error in pmm_tswizard.log file:
“Unable to find image on **Parallels NetBoot** servers”.
Incorrect network configuration across subnets.
For further investigation, there must be the Wireshark application (Download Wireshark) installed on Parallels Netboot server and on a Mac or Windows computer in the same subnet as the target Mac.
The screenshots demonstrate the examples of network packets that should pass through the network environment between the Mac and the NetBoot server:
When a Mac starts with Option+N keys pressed a search for NetBoot server starts.
The Mac screen should display a list of the images available for download.
When you choose an image, the boot of minimal OS starts over TFTP.
Then the NetInstall.dmg file is downloaded via HTTP using port 80.
The search of an OS Image for deployment starts at the step ‘Apply OS Image’ in Task Sequence.
That's how the link for NetRestore image is sent to the Mac:
And that's how it starts downloading:
You may refer to the articles for more information: