HI Ralf I don't know what happened to my previous reply, I never got a copy (It was witten on my tablet because I was working in the house). Here is an approx copy. I have yet to learn much about a systemd-nspawn container but I'll try. I didn't nkow what syslinux was supposed to do but I'll research it now. Many thanks Steve
there are several possible reasons to get this error. One reason could be installing grub, if you try it from a Linux running in a systemd-nspawn container. IOW also mention from where you try to install grub.
Btw. a while back I migrated from grub to syslinux for my Linux multi-boot machine, this means no Windows, no FreeBSD, no UEFI or anything else, just several Linux installs. FWIW I'm still using MBR. I like syslinux much better than grub, but it might not fit to the taste of everybody.
Regards, Ralf