The type or namespace name AccountManagement does not exist in the namespace System.DirectoryServices

Error occurs due to missing an assembly reference.

Add the Reference System.DirectoryServices.AccountManagement and you'll be good to go. Following should be appear under assemblies tag in web.config once you add the reference.

<add assembly="System.DirectoryServices.AccountManagement, Version=4.0.0.0, Culture=neutral, PublicKeyToken=B77A5C561934E089"/>