# Create user runtime directory
sudo install -d -o www-data -g www-data -m 0700 /run/user/`id -u www-data`

# Start user manager
sudo systemctl start user@`id -u www-data`

# Optionally enable lingering (User manager is spawned for the user at boot and kept around after logouts).
sudo loginctl enable-linger www-data

# Test it
sudo -u www-data \
    XDG_RUNTIME_DIR=/run/user/`id -u www-data` \
    DBUS_SESSION_BUS_ADDRESS=unix:path=/run/user/`id -u www-data`/bus \
    systemd-run --user echo "foo-`id -u`-bar"

# See if it's working
sudo systemctl status user@`id -u www-data`

# If not working try restarting this services:
sudo systemctl restart dbus.socket
sudo systemctl restart dbus.service
sudo systemctl restart user@`id -u www-data`