In 2017 Apple came out with iMac Pro with T2 chip. In this Mac, Apple has removed an option to boot from a network location (so called NetBoot) and added hardware encryption to disk drive(so called 'secure enclave'). In 2018 Apple released Macbook Pro, Macbook Air and Mac Mini with the same parameters.
To be able to boot a Mac with T2 Chip from PMM USBBoot:
1. PMM USBBoot should be captured from a T2 Mac with Mojave or Catalina installed.
2. Secure Boot should be set to 'No Security' and External Boot to 'Allow booting from external media' per article.
NetRestore (System) image:
1. Due to 'secure enclave' on T2 Macs (hardware encrypted disk drive) a NetRestore (System) image should be captured from a Non-T2 Mac.
2. The minimum OS requirement for T2 Macs is 10.13.6+ Supplemental Update 1.
Summarizing points 1 and 2, we recommend capturing NetRestore (System) images from Non-T2 Macs with Mojave or Catalina installed (since 10.13.6+ Supplemental Update 1 could not be installed on a Non-T2 Mac).
If you do not have any non-T2 Macs we may suggest installing Parallels Desktop, spinning up a macOS Virtual Machine and capturing the NetRestore (System) image from it.