coreos-installer (and Fedora CoreOS) can’t install to a partition; it needs an entire disk. It should be possible to install, though; it seems like this might be a bug in the installer.
It might be relevant that the partitions are backed by LVM. I’ve just tried that locally and was able to install successfully, but my setup might be different from yours.
Okay, fallback plan for strace. Within the live system, you can manually extract the strace rpm:
curl https://kojipkgs.fedoraproject.org/packages/strace/5.10/1.fc33/x86_64/strace-5.10-1.fc33.x86_64.rpm > strace.rpm
rpm2cpio strace.rpm | cpio -i --make-directories
# note no leading slash
sudo usr/bin/strace coreos-installer install /dev/vda