This is wrong. It should return your username:
artix-live:[artix]:~$ grep autologin /etc/group
autologin:x:976:artix
This /etc/sddm.conf is from a live Artix Cinnamon ISO. Setting Session and User should be enough to allow autologin, provided User is member of the autologin group.
[Autologin]
# Whether sddm should automatically log back into sessions when they exit
Relogin=false
# Name of session file for autologin session (if empty try last logged in)
Session=cinnamon.desktop
# Username for autologin session
User=artix
[General]
# Halt command
HaltCommand=/usr/bin/loginctl poweroff
# Input method module
InputMethod=qtvirtualkeyboard
# Initial NumLock state. Can be on, off or none.
# If property is set to none, numlock won't be changed
# NOTE: Currently ignored if autologin is enabled.
Numlock=none
# Reboot command
RebootCommand=/usr/bin/loginctl reboot
[Theme]
# Current theme name
Current=artix
# Cursor theme used in the greeter
CursorTheme=
# Number of users to use as threshold
# above which avatars are disabled
# unless explicitly enabled with EnableAvatars
DisableAvatarsThreshold=7
# Enable display of custom user avatars
EnableAvatars=true
# Global directory for user avatars
# The files should be named <username>.face.icon
FacesDir=/usr/share/sddm/faces
# Theme directory path
ThemeDir=/usr/share/sddm/themes
[Users]
# Default $PATH for logged in users
DefaultPath=/usr/local/bin:/usr/bin:/bin
# Comma-separated list of shells.
# Users with these shells as their default won't be listed
HideShells=
# Comma-separated list of users that should not be listed
HideUsers=
# Maximum user id for displayed users
MaximumUid=60000
# Minimum user id for displayed users
MinimumUid=1000
# Remember the session of the last successfully logged in user
RememberLastSession=true
# Remember the last successfully logged in user
RememberLastUser=true
# When logging in as the same user twice, restore the original session, rather than create a new one
ReuseSession=false
[Wayland]
# Enable Qt's automatic high-DPI scaling
EnableHiDPI=false
# Path to a script to execute when starting the desktop session
SessionCommand=/usr/share/sddm/scripts/wayland-session
# Directory containing available Wayland sessions
SessionDir=/usr/share/wayland-sessions
# Path to the user session log file
SessionLogFile=.local/share/sddm/wayland-session.log
[X11]
# Path to a script to execute when starting the display server
DisplayCommand=/usr/share/sddm/scripts/Xsetup
# Path to a script to execute when stopping the display server
DisplayStopCommand=/usr/share/sddm/scripts/Xstop
# Enable Qt's automatic high-DPI scaling
EnableHiDPI=false
# The lowest virtual terminal number that will be used.
MinimumVT=7
# Arguments passed to the X server invocation
ServerArguments=-nolisten tcp
# Path to X server binary
ServerPath=/usr/bin/X
# Path to a script to execute when starting the desktop session
SessionCommand=/usr/share/sddm/scripts/Xsession
# Directory containing available X sessions
SessionDir=/usr/share/xsessions
# Path to the user session log file
SessionLogFile=.local/share/sddm/xorg-session.log
# Path to the Xauthority file
UserAuthFile=.Xauthority
# Path to xauth binary
XauthPath=/usr/bin/xauth
# Path to Xephyr binary
XephyrPath=/usr/bin/Xephyr