Problem with syncing images with a new computer (Lightroom Mobile > Lightroom Classic)

My issue is with syncing images between Lightroom Mobile (on iPhone) and Lightroom Classic on a new iMac which replaced my old iMac - this seems to have confused things.

For images that were synced on my old iMac I now get: "IMG_1234.JPG" could not be used because the original file could not be found. Would you like to locate it? message. I can't find a way to tell Lightroom to forget about the original file (it's gone with my old computer) and to download the original file on my new computer.

I've tried deleting all synced photos and starting again - but when I re-sync, I get the same result:

• If the photo was previously synced on my old iMac, it 'syncs' (see screenshot), but because the original file isn't there, and it won't download a new file, I'm stuck with a grey thumbnail.
• New photos are fine.

Any help would be much appreciated.