How to add a Password to User Account if previously it set to "Log In Without Password"
Solution 1:
Open a terminal, and enter "passwd" ... this will change the password for the current user. If you want to change an other users password, you will need to enter "sudo passwd", because then you need to run passwd with root privileges.
Solution 2:
just open a terminal and write passwd It will ask for the new password if it was blank previously or ask for the old password if it was not blank.