When downloading, you will encounter two main file extensions for loadable content:

For years, Autodesk community experts and employees have provided direct links to independent content libraries for various Revit versions, including 2018. These links are a lifesaver for users who cannot find the content on the main Autodesk site.

The most reliable way to obtain the Revit 2018 library is through your official .

This error means Revit cannot find the folder containing its family templates (.rft files), often because it was never installed or the path is wrong.