The latest parted magic iso image I can find (free) is 2013. So I looked through that iso and based on what I see I have an observation.
The error message mentions unable to mount the squashed file system file.
I don’t see where you are transferring that file system via iPXE?
I do see that file on the iso in /pmagic/pmodules directory.
I would be interested in reviewing the following files from the 2018 iso image.
/boot/syslinux/syslinux.cfg
/boot/pxelinux/sample_pxelinux.cfg
Lastly are you trying to iPXE boot this image on a uefi or bios based image?