systemctl, how to unmask

The commands you are using are both correct. See also the manual.

It seems the unmask command fails when there is no existing unit file in the system other than the symlink to /dev/null. If you mask a service, then that creates a new symlink to /dev/null in /etc/systemd/system where systemd looks for unit files to load at boot. In this case, there is no real unit file.

Others seem to have similar issues

x11-common.service was also masked on my system. You can fix it like this:

First check that the unit file is a symlink to /dev/null

file /lib/systemd/system/x11-common.service

it should return:

/lib/systemd/system/x11-common.service: symbolic link to /dev/null

in which case, delete it

sudo rm /lib/systemd/system/x11-common.service

Since you changed a unit file, you need to run this:

sudo systemctl daemon-reload

now check the status:

systemctl status x11-common

if it doesn't say loaded and running (if the circle is still red), reinstall the package:

sudo apt-get install --reinstall x11-common

and reload the daemon again

sudo systemctl daemon-reload

and check status once more

systemctl status x11-common

Now it's green and running :) The service has no systemd unit file, but systemd happily uses the script for it in /etc/init.d instead.

It could be your service has an empty override file, like this:

● redis-server.service - Advanced key-value store
   Loaded: loaded (/lib/systemd/system/redis-server.service; masked; vendor preset: enabled)
  Drop-In: /etc/systemd/system/redis-server.service.d
           └─limit.conf

Check if limit.conf is an empty file. If it is, please remove it. Then the service should be unmasked.