Installing screen savers is much like installing any other piece of software. You must be logged in as a user that has rights to install software and make basic changes to the system directory.

Usually, this means being logged in as an admin user. Restricted users do not have the rights to install software, so most often when logged in as a restricted user the installation will fail.

To remedy the situation, have an administrator install the screen saver or see if your user privileges can be upgraded.