Lightroom Auto Import Destination: existing folder not accepted
I am running Lightroom Classic 14.3.1 on macOS Sequoia 15.4.1. I want to be able to copy photos from the Photos App into Lightroom using the Auto-Import feature. In the Auto-Import settings, I define the Watched Folder with no problem. Photos does export images to this folder just fine.
I then move to define the Destination Folder, stored as a Lightroom-indexed folder.

When I select OK, I get this error

I have checked the Adobe docs at https://helpx.adobe.com/lightroom-classic/help/import-photos-automatically.html and in the "Select a destination folder for auto-imported photos" section it says "To select an existing destination folder, select the folder." I have tried all manner of destination folders, but always get the same error:
* within LR indexed folder tree
* outside tree
* on external drive
* on internal drive
What am I doing wrong?
