Why does the Project Asset Inspector show I'm missing textures?

A common misconception is that Cinema 4D's built in Project Asset Inspector works with third party renderers. This can lead to confusion. Depending on the third party renderer you're using, you'll need to use the custom texture inspector built by the developer of that renderer:

In Redshift, use the Redshift Asset Manager.

In Octane, use the Octane Texture Manager.

In Arnold, use the Assets/ Tx Manager