You don’t use the RAID Assistant to convert the RAID. The device you purchased is a hardware RAID device. The RAID Assistant in Disk Utility is for software RAIDs.
You need our configurator software here: G-RAID Configurator
This is the same instructions I posted in the other thread you posted in. https://community.wd.com/t/12tb-g-raid-out-of-the-box-wont-setup-as-raid-1-grayed-out-mac-sierra/202606/7
