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Usually after the installation, the folder "Accessories Library xx" ends up in the wrong place.
Go to your ArchiCad folder (C:\Program Files\GRAPHISOFT\ARCHICAD xx\) and see if there's a folder called "Accessories Library xx" (the "xx" is your ArchiCad version number). If so, just move or copy that "Accessories Library xx" folder into "ARCHICAD Library xx" folder.
Archicad 21 Windows 10, Corei7 3.4 GHz, 16 GB Ram.