Hmm interesting the syntax looks correct I just did the same thing to ensure I didn't give you a mistyped command.
The only way I could replicate your issue is by specifying an incorrect domain name.
Ensure that's correct (check the name at the top of the tree in AD Users and Computers), heres some examples:
For a .local domain
ou=test,dc=mydomain,dc=local
For a .com domain
ou=test,dc=mydomain,dc=com
For a .co.uk domain
ou=test,dc=mydomain,dc=co,dc=uk
Hope that helps.