Comment 3 for bug 1873797

I also get this problem, on a minimal Ubuntu installed on an LVM-managed disk. However, I believe that the fix should be:

sudo sed -i 's,\(ln -s "\$0" /tmp/mountroot-fail-hooks\.d/\)"\$0",\1"\$1",' /usr/share/initramfs-tools/scripts/functions

i.e, that the path in /tmp/mountroot-fail-hooks should be given by the parameter to the funcion. Apply this fix with (notice that bug 1835660 may now occur):

sudo update-initramfs -u

You can verify that the message goes away with

sudo journalctl -b | grep mountroot-fail-hooks

after next reboot.