Problem to create RaidZ

can you post the complete task log? (double click on the zfs create task log in the bottom)
 
# /sbin/zpool create -o ashift=12 DATEN raidz /dev/disk/by-id/ata-WDC_WD20EFRX-68EUZN0_WD-WCC4M4AJA9HA /dev/disk/by-id/ata-WDC_WD20EFRX-68EUZN0_WD-WCC4M2HKN9PD /dev/disk/by-id/ata-WDC_WD20EFRX-68EUZN0_WD-WCC4M0RSC01N /dev/disk/by-id/ata-WDC_WD20EFRX-68EUZN0_WD-WCC4M5DRJ2PU
invalid vdev specification
use '-f' to override the following errors:
/dev/disk/by-id/ata-WDC_WD20EFRX-68EUZN0_WD-WCC4M5DRJ2PU contains a corrupt primary EFI label.
TASK ERROR: command '/sbin/zpool create -o 'ashift=12' DATEN raidz /dev/disk/by-id/ata-WDC_WD20EFRX-68EUZN0_WD-WCC4M4AJA9HA /dev/disk/by-id/ata-WDC_WD20EFRX-68EUZN0_WD-WCC4M2HKN9PD /dev/disk/by-id/ata-WDC_WD20EFRX-68EUZN0_WD-WCC4M0RSC01N /dev/disk/by-id/ata-WDC_WD20EFRX-68EUZN0_WD-WCC4M5DRJ2PU' failed: exit code 1
 
you could try the 'wipe disk' button in the gui (node -> disks)

or use a cli tool of your choice to remove the partitions/wipe the disk (e.g. fdisk/gdisk, wipefs, etc. this is general linux stuff and not really proxmox specific)
 
yes that is only available in pve 7