Most probably this is a centralized user. To confirm, run the following command:
grep -c "DXserver Administrator" /etc/passwd
If the user is present locally on the server, it should return '1'.
If the user is centralized, it should return '0'.
If you get '0' as an output, possibilities are you are installing version 12.0 SP15 or higher where installing as non-root user is supported but that non-root user has to be local. Centralized user is not support in 12.0 version of CA Directory while the support for this enhancement was added in 12.6 release. So the solution would be to use CA Directory 12.6.x version if you cannot have this "DXserver Administrator" defined in /etc/passwd file.