I disabled password for login at User Account setting, and later when I ran sudo, I get "Sorry, try again." It seems my previous password doesn't work anymore. I also tried hitting enter directly but it didn't work either.
I attempted to change the root password by following this article: https://linuxconfig.org/how-to-reset-lost-root-password-on-ubuntu-16-04-xenial-xerus-linux. Everything works fine, I did have got the "update password successfully" message. However, when I get back to the graphic desktop and tried to sudo anything, the new password didn't get me any more luck.
I repeated the process three more times and made sure there is no typo in setting new password. I also tried to remove sudo requirement by following this article: https://www.cyberciti.biz/faq/linux-unix-running-sudo-command-without-a-password/ after resetting password in grub mode, but that didn't help.
So right now the OS works fine but I can't do sudo anymore or authenticate anything at all. Any help will be appreciated!
So appearantly sudo asks for the user's instead of the root password, I guess that's while changing root password doesn't help. (Please correct me if that's not exact). However, I still can't find anything to get sudo to work after trying every single password I ever set up..