Failed to intstall ubuntu18.04.1/16.04.5 on RSTe raid mode in legacy mode

Install ubuntu 18.04.1 on RSTe raid5 under Legacy mode, it can detect MDADM container（Intel/DDF RAID） successfully.Activated MDADM container and Serial ATA RAID device (screenshot1-3) to go ahead installing. But it prompted error that unable to install GRUB in /dev/mapper/ubuntu--vg-root when excuting 'grub-install /dev/mapper/ubuntu--vg-root'(screenshot4).

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1.Install ubuntu18.04.1 on RSTe raid0 in Legacy mode, issue also exists.
2. Install ubuntu18.04.1/16.04.5 on RSTe raid5 in UEFI mode, issue not exists.
3. Install ubuntu16.04.5 on RSTe raid5 in Legacy mode, it prompted error that mount media failed after booted into kernel(screenshot5), but can go ahead automatically. And it can install Grub successfully, but cannot login to OS after reboot(screebshot6-7).