Since Archicad 20 is an older version (current versions are v26/v27), you must ensure the objects you download are backward compatible or saved in a format v20 can open.
Download Parametric Objects for Archicad | Graphisoft | United States
ArchiCAD 20 , high-quality library objects are available for free through several established BIM and CAD platforms. While ArchiCAD 20 is an older version, many GDL (Geometric Description Language) objects remain backward-compatible or are specifically archived for legacy use. Graphisoft Community Top Sites for Free ArchiCAD Objects
If you see this error: