Screensavers made with iScreensaver have two parts, a .SCR file and a “Libs” folder. This error happens when you’ve misplaced, lost, deleted, or renamed the Libs folder.
It can also happen if you try to run the installer from within a Zip file. In that case the solution is to fully unzip the files first. (On most versions of Windows, right-click the Zip file and choose “Extract All”)