I know that this is deadly old, but for some others that are coming here with the same problem, in my case the solution was to enter in BIOS and activate mode RAID on, changing it from the previous AHCI mode.
With AHCI mode it was working under Ubuntu 16.04 with mdadm 3.3, however in 18.04 mdadm is version 4.1 and according to man page http://manpages.ubuntu.com/manpages/xenial/man8/mdadm.8.html:
A key value of IMSM metadata is that it allows interoperability with boot ROMs on
Intel platforms, and with other major operating systems. Consequently, mdadm will
only allow an IMSM array to be created or modified if detects that it is running on
an Intel platform which supports IMSM, and supports the particular configuration of
IMSM that is being requested (some functionality requires newer OROM support).
These checks can be suppressed by setting IMSM_NO_PLATFORM=1 in the environment.
This can be useful for testing or for disaster recovery. You should be aware that
interoperability may be compromised by setting this value.
Apparently RAID ON is needed to the new version of mdadm to detect a platform that supports IMSM.