Is SharePoint 2010 compatible with Windows 2008 R2 Server Core

You appear to be missing the required FarmCredentials parameter to the cmdlet.

If that doesn't help, you may need to run the Powershell console as administrator to get the command to work properly.

http://technet.microsoft.com/en-us/library/ff607838.aspx

Did you try to add a custom membership provider? LIke in the following article:

http://www.setfocus.com/technicalarticles/articles/MOSS2007-and-Novell-LDAP-Authentication.aspx