When the Maven Site plugin deploys a site to Nexus, it will need to supply the appropriate deployment credentials to Nexus. To configure this, you will need to add credentials to your Maven Settings. Open up your ~/.m2/settings.xml and add the following server configuration to the servers element. If you already have a servers element, just add a new server element with the appropriate id, username, and password.

<settings>
  <servers>
    <server>
      <id>nexus-site</id>
      <username>deployment</username>
      <password>deployment123</password>
    </server>
  </servers>
 </settings>