I can not add domain !!!

I installed Webmin + Virtualmin on Debian 4.0
I have checked configuration.

The status of your system is being checked to ensure that all enabled features are available, that the mail server is properly configured, and that quotas are active …

  BIND DNS server is installed.

  Mail server Postfix is installed and configured.

  Apache is installed.

  Webalizer is installed.

  MySQL is installed and running.

  ProFTPd is installed.

  Logrotate is installed.

  Using network interface eth0 for virtual IPs.

  Default IP address for virtual servers is 85.xx.xx.xxx.

  Quotas are not enabled on the filesystem / which contains home directories under /home and email files under /var/mail. Quota editing has been disabled.

  Shell /bin/false for FTP users is not included in /etc/shells, which may prevent FTP access.

  All commands needed to create and restore backups are installed.

… your system is ready for use by Virtualmin.

but it can not add domain and gives this Error.

Setting Up Virtual Server
In domain domain.com
Creating home directory …
… done

Creating administration group domain …
… done

Creating administration user domain …
… done

Failed to create virtual server : Cannot write to directory

where is problem?
How I can fix it?