It sounds like Lightroom Classic (LrC) is having trouble synchronizing a single image, which is preventing you from synchronizing a new collection of images. Here are a few troubleshooting steps you can try:
Check the sync status panel: In LrC, click on the cloud icon in the top right-hand corner of the screen to open the sync status panel. This will show you the status of all your synced collections and any errors that may be preventing synchronization.
Check your Lightroom Cloud account: Log in to your Lightroom Cloud account on the Adobe website to see if the image in question is there. If it is, you may be able to locate it by searching for the file name or metadata.
Reset sync data: You can try resetting your Lightroom sync data to see if that resolves the issue. To do this, go to Preferences > Lightroom Sync and click on the "Reset Sync Data" button. This will clear out any sync errors and force a resync of your Lightroom catalog.
Check for conflicting metadata: If the image in question has conflicting metadata (e.g. the same file name as another image in your catalog), Lightroom may be having trouble syncing it. Try searching for the file name in Lightroom and see if any other images come up with the same name.
Check for file corruption: It's possible that the image file itself is corrupted or damaged, which could be preventing Lightroom from syncing it. Try opening the file outside of Lightroom to see if it opens properly. If it doesn't, you may need to restore the file from a backup or try repairing it using file recovery software.
Hopefully one of these steps will help you locate and resolve the sync issue with the problematic image in your Lightroom catalog.
Let me know how things are going,