Check the status of your library in Photos for macOS

1. Select Library in the Photos sidebar.
2. Click All Photos in the toolbar at the top of the window.
   
3. Scroll to the bottom of the window to see status information:
   • The number of photos and videos that are in your library.
   • When your library was last updated with iCloud Photos, and the option to click Pause or Resume if updates are in progress.
   • The number of items that couldn't sync to iCloud. Click View for more details.
   • The number of items — if any — that are on this Mac only and not available to your other devices.

To see which items couldn't sync to iCloud, click the View button next to the message at the bottom of the Library window or click Unable to Upload in the sidebar. You can try to reimport the items from the Unable to Upload album into your Photos library to sync the items with iCloud Photos. 

1. Press Command-A to select all of the items in the Unable to Upload album, choose File > Export, then choose Export Unmodified Original For [number] Photos.
   
2. In the dialogs that follow, click Export, choose a save location, then click Export Originals.
3. After the items export, make sure they are all still selected in the album, then choose Image > Delete [number] Photos or Videos.
4. Click Delete, then click OK.
5. Choose the Recently Deleted album in the sidebar, then choose Delete All. Or delete only the items that you just deleted. 
6. Choose File > Import, then choose the images you previously exported. For any items successfully synced to iCloud Photos, edits or keywords that you added to the affected items might be lost.

If you set up Photos to allow referenced files, which are photos and videos stored outside of your library, Photos counts those as “Items on This Mac Only.” If you want to consolidate those items into your library so that they can be uploaded to iCloud Photos and available to your other devices, follow these steps:

1. From the menu bar in Photos, choose File > New Smart Album.
2. Name the Smart Album “Referenced” and set the conditions to “Photo is referenced.” Click OK.
   
3. Select the Referenced album in the sidebar.
4. Press Command-A to select all items in the album, then choose File > Consolidate.