Are you doing this in remote session via ssh, by any chance?
If yes, dbus is most likely not running in the remote ssh session. You can start it yourself by adding “dbus-launch” in front of the command:
dbus-launch dconf write /org/mate/terminal/profiles/profile6/visible-name "'kerpl'"
Edited to add: If you are doing this in a remote ssh session and you need to change settings for another user, just add “sudo -u otheruser” before the dbus-launch.