[mdmonitor] The unit files have no installation config

In chapter Raid p.4, the command sudo systemctl start mdmonitor goes well but then sudo systemctl enable mdmonitor returns:

The unit files have no installation config (WantedBy=, RequiredBy=, Also=,
Alias= settings in the [Install] section, and DefaultInstance= for template
units). This means they are not meant to be enabled using systemctl.

Possible reasons for having this kind of units are:
• A unit may be statically enabled by being symlinked from another unit's
.wants/ or .requires/ directory.
• A unit's purpose may be to act as a helper for some other unit which has
a requirement dependency on it.
• A unit may be started when needed via activation (socket, path, timer,
D-Bus, udev, scripted systemctl call, ...).
• In case of template units, the unit is meant to be enabled with some
instance name specified.

Did anything go wrong? The /etc/mdadm.conf file has:

ARRAY /dev/md0 metadata=1.2 name=ip-a-b-c-d:0 UUID=c0e6f53c:4df9342f:bbbc4aca:dc47709e
MAILADDR my@email.com