When uploading projects for culling and editing, we highly recommend using Smart Previews, as they are a great option.

However, it's important to note that when uploading high-resolution cloud photo backups, Smart Previews are not compatible. To successfully upload these photos to the Imagen cloud backup, it's necessary to connect all original photos to your Lightroom Classic catalog

Discover further information about this issue and its resolution in the subsequent articles:
Imagen Cloud backup Error: Imagen can't locate my original photos

How to Identify And Re-Connect Missing Original Photos in Lightroom Classic Catalogs