It's just message , not error.
/etc/kernel/proxmox-boot-uuids is used for ZFS install
Couldn't find EFI system partition is excepted if install is Legacy BIOS.
I have the same problem, unsure what to add in /etc/kernel/proxmox-boot-uuids
cat /etc/kernel/proxmox-boot-uuids
cat: /etc/kernel/proxmox-boot-uuids: No such file or directory
root@pbs:~# lsblk -f
NAME FSTYPE FSVER LABEL UUID...