I have had the same problem as the rest of you: stuck on 36 images that wouldn't sync and didn't seem to exist. I'm using Windows (which I believe is relevant in my case) and for me, the problem turned out to be related to images that I had taken with my iPhone, just after Apple introduced the HEIC-format (something which I only realised weeks after they did it). I flagged all images in Lightroom Classic, then moved to Lightroom CC and filtered the unflagged images, i.e. the ones that wouldn't sync to my sync folder in Lightroom Classic. They were all HEIC-files from my mobile. So I just made sure I had those images backed up somewhere else before I simply deleted them in Lightroom CC. For me this solved the problem and Lightroom Classic is no longer trying to sync images that doesn't seem to exist. Reading your posts, I doubt this is the solution for everyone, but perhaps some of you might want to have a go at it.
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