This issue Most commonly occurs if you're in the process of transferring your domain away from Yahoo! While your transfer is pending, you cannot change your name servers. As soon as your transfer has completed, you can manage your name servers with your new provider.
Another reason that you may be getting this error, is due to a typo. If you are getting this error, we suggest that you contact your hosting provider and re-verify the name servers that you were given
The last and most uncommon reason for this error is due to the name servers not being registered. Once you have confirmed the name servers, you may need to contact customer support to have your name servers registered. Before you contact customer support be ready to provide the IP address assigned to each name server you need to register.