Hi,
That's due to a Joomla plugin you have activated: in the Joomla backend go to the Plugin Manager and search for the plugin "User - Contact Creator". You have it enabled, so it tries to automatically create a Joomla contact (accessible under -> Components -> Contacts) for each new user registering.
Unless you want to publish a (contact) list of all your users, maybe even incl contact form etc, you don't need this, so you should disable the plugin. New users registering will then not see the error anymore.
The good news is that, due to the error/"misconfiguration", at least you don't have a long list of (useless) contacts in your database. Again, check under -> Components -> Contacts, there should be only the ones you entered manually, if any.