There are three errors that people commonly run into when trying to update a domain dynamically.

Unauthorised Host indicates that the domain does not have dynamic updates enabled. If you login to the members page you will see "dynamicDNS is: disabled" below the domain name. Click "disabled" and follow the steps described in setup to enable dynamic updates for the domain.

Invalid Credentials means that the login information is wrong. Double check that you have used the same login information you use to access the members page in the client. If that does not work you may be dealing with a third level domain as discussed under TLDs.

Updates too frequent indicates that you have tried to update the domain more than once in a ten minute period. This often comes up in testing, when people will try to update the domain repeatedly to make sure it's working, or if the client has a setting for how frequently to send an update which is set to less than 10 minutes. If you try this just wait 10 minutes until your next update or look through your client to find any frequency setting which is set to less than 10 minutes.