Raid installs aren't supported by ubiquity. As a workaround you can install the mdadm package and configure the raid yourself ( and I assume you did this ), but you must also then install the package in the target system as well and install grub yourself.
** Changed in: grub-installer (Ubuntu) Status: New => Invalid -- You received this bug notification because you are a member of Ubuntu Bugs, which is subscribed to Ubuntu. https://bugs.launchpad.net/bugs/1765514 Title: Doing the 18.04-beta-2 install using RAID5 root partition. GRUB2 fails to write to disk. To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/grub-installer/+bug/1765514/+subscriptions -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs