Quick summary

Imagen couldn't read the photo information from your Lightroom Classic catalog, so the upload couldn't start.

How to fix this

1. Fully close Lightroom Classic. Check Activity Monitor (Mac) or Task Manager (Windows) to make sure it's no longer running.
2. Make sure the catalog is on your computer's local drive, not a network drive, external drive or cloud-synced folder (Dropbox, iCloud, OneDrive, Google Drive). If it isn't, move it to a local folder and open it from there.
3. Make sure the drive that holds your photos is connected.
4. Restart Imagen and create the project again.
5. If it still fails, the catalog may be damaged. In Lightroom Classic, open the catalog and go to File › Optimize Catalog. If Lightroom offers to repair it, accept. Then try again.