Edit the configuration file /etc/lightdm/lightdm.conf manually. From the terminal run this command to edit that file:
gksudo xed /etc/lightdm/lightdm.confUnder the
[SeatDefaults] heading change there should be a line autologin-user=
(it may have a # in front of the line, remove that character to
activate the line). Add your username (in lower case) after the = on
this line (if you're unsure of your username run whoami on a terminal).https://forums.linuxmint.com/viewtopic.php?t=250723