Can't login to a linux server with new user
Look into (OSX: /var/log/secure.log, Debian: /var/log/auth.log) after you attempt to log in. What is the error?
Have you made sure he has an accessible home directory to log into?
Your newly created user would also need a valid shell listed in /etc/shells.
getent passwd <username> | cut -d: -f7
Should yield sometihing like /bin/bash, /bin/sh, etc. (something listed in /etc/shells).
To change the shell for your newly created user could be done by using the chsh command, example to :
chsh -s /bin/bash <username>