Executing the command manually reveals the reason:
root@box:/var/log/proxmox-backup/tasks# proxmox-backup-manager disk zpool create zpool1 --devices sdb,sdc,sdd --raidlevel raidz --add-datastore
create RaidZ zpool 'zpool1' on devices 'sdb,sdc,sdd'
# "zpool" "create" "-o" "ashift=12" "-m" "/mnt/datastore/zpool1" "zpool1" "raidz" "sdb" "sdc" "sdd"
TASK ERROR: command "zpool" "create" "-o" "ashift=12" "-m" "/mnt/datastore/zpool1" "zpool1" "raidz" "sdb" "sdc" "sdd" failed - status code: 1 - invalid vdev specification
use '-f' to override the following errors:
/dev/sdb contains a corrupt primary EFI label.
/dev/sdd contains a corrupt primary EFI label.
root@box:/var/log/proxmox-backup/tasks# 
