The status message is showing "1 item on this Mac only".
This usually means, that you have an item in your Photos Library that is blocking the syncing, because it is incompatible with iCloud, or damaged, or is having a missing original. Your other devices are syncing well, because this item has not been uploaded and hopefully never will. You will have to identify this item.
Are you seeing an album folder in the sidebar named "unable to upload"? Look there for a problematic item.
You can also test with a smart album as described here: Check the status of your library in Photos for macOS - Apple Support
The "on this Mac only" message could be caused by referenced items, that have not been copied into the library. iCloud Photos does not work with referenced files. Then consolidating will help. If the files cannot be consolidated, try to export them and delete them from the library.
"If you see "Items on This Mac Only"
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:
- From the menu bar in Photos, choose File > New Smart Album.
- Name the Smart Album “Referenced” and set the conditions to “Photo is referenced.” Click OK.

- Select the Referenced album in the sidebar.
- Press Command-A to select all items in the album, then choose File > Consolidate.
"