Hello,
Error 6 means that permission is denied for the user. If you say that you enter the proper credentials then we might need some more info to figure out what goes wrong. If you could send us to issues [at] nomachine [dot] com the following information from your CentOS host:
1) /etc/pam.d/nx, /etc/pam.d/su, /etc/pam.d/system-auth, /etc/pam.d/sshd, /etc/pam.d/password-auth files
2) output of the command (after reproducing the issue):
sudo grep -r nxexec /var/log/*
Thank you.