In the downloaded rar file there must be a picture object: natural.jpg or natural.png or some other image.
The library object is not finding this file in any of the loaded libraries and so gives the error message.
This picture file must be in one of the linked libraries set in the Library manager.
If it is not in the rar file you downloaded then probably the uploader person of that file was not careful enough and did not include all needed files in the rar file.
Loving Archicad since 1995 - Find Archicad Tips at x.com/laszlonagy
AMD Ryzen9 5900X CPU, 64 GB RAM 3600 MHz, Nvidia GTX 1060 6GB, 500 GB NVMe SSD
2x28" (2560x1440), Windows 10 PRO ENG, Ac20-Ac27