User was attempting to extend the Active Directory Schema but when using ADSSchema.exe the following error occurred even though they were a Schema Admin.
Install the Active Directory Schema snap-in to the MMC console (see link below for details on how to do this). In the left hand side of the console, right click on Active Directory Schema and select Operations Manager. Enable the âSchema may be modified on this Domain Controllerâ check box. Click Ok. Schema Admins now have the ability to extend the schema.
For more information on installing the Active Directory Schema snap-in and setting the modification rights, see the article 285172 from Microsoft.
The user was attempting to extend the schema on a domain controller where schema modifications were disallowed.