mjhjmtu

I have 2 files in /sys/class/backlight/ - acpi_video0 and intel_backlight. intel_backlight is the one which controls my screen brightness (echo/cat), however acpi_video0 is the default.

By default the service systemd-backlight@acpi_video0.service is enabled which should save/restore brightness at shutdown/startup, but it doesn't work since it points to the wrong file.

When trying to enable systemd-backlight@intel_backlight.service instead, but I get this message:

The unit files have no [Install] section. They are not meant to be enabled
using systemctl.
Possible reasons for having this kind of units are:
1) A unit may be statically enabled by being symlinked from another unit's
 .wants/ or .requires/ directory.
2) A unit's purpose may be to act as a helper for some other unit which has
 a requirement dependency on it.
3) A unit may be started when needed via activation (socket, path, timer,
 D-Bus, udev, scripted systemctl call, ...).

How do I fix this?

It may be that you're just missing the backlight: part of the command:

# systemctl enable systemd-backlight@backlight:intel_backlight.service

You may also need to escape the :, i.e.:

# systemctl enable systemd-backlight@backlight:intel_backlight.service

Since you have two folders in /sys/class/backlight/, you probably have two graphics cards. The Arch wiki suggests adding one of the following kernel parameters in your bootloader:

acpi_backlight=video
acpi_backlight=vendor
acpi_backlight=native
acpi_backlight=none