This is how I resolved the issue when Lightroom Classic would not complete the sync of photos or metadata.

Absolutely make a backup of your Lightroom catalog before proceeding.

The first thing to try is “Rebuild Sync Data”.

Contents

Rebuild Sync Data (Level 1)

Navigate to: Edit > Preferences (Win) or Lightroom Classic > Preferences (macOS)
Click the Lightroom Sync tab.
Press and hold the Alt key, and click the “Rebuild Sync Data” button.

Methods to try (Level 2)

Exit and restart Lightroom Classic
Reboot the computer
Pause sync, then Start sync
Stop “Core Sync” in Task Manager
Turn off “Sync to Lightroom” on the Collection referencing the photos being synced.
Rebuild Sync Data, again
- Edit > Preferences (Win) or Lightroom Classic > Preferences (macOS)
- Click the Lightroom Sync tab.
- Press and hold the Alt key, and click the “Rebuild Sync Data” button.

Start syncing everything again.

Go to Edit > Catalog Settings.
On the “General” tab, Set “Backup up catalog” to “Every time Lightroom exits”
Exit Lightroom, and make sure to click “Back up” at the Back Up Catalog prompt.
Go to the directory/folder where your Lightroom Catalog is stored.
Rename the file which starts with the name of your Catalog, and ends with “Sync” or “Sync.lrdata” by adding zzz to the beginning for the filename.
Start Lightroom Classic. A new “*Sync.lrdata” file will be created.

Lightroom Classic should start syncing everything with “Sync to Lightroom” enabled. Wait a while for the sync to complete.

Do not add or remove sync objects until the initial sync is complete.

You can now delete the “Sync.lrdata” with a filename starting with zzz.

Program:	Lightroom Classic r11.2 (Camera Raw 14.2)
OS	Windows 10 v21H2
Date Resolved	2022-03-06