Photos not Uploading to iCloud on Mac

See the article Update macOS on Mac - Apple Support

Set up and use iCloud Photos - Apple Support

Pause library uploads to iCloud

When you turn on iCloud Photos, the time it takes for your photos and videos to upload to iCloud depends on the size of your collection and your internet speed. If you have a large collection of photos and videos, your upload might take more time than usual. You can see the status and pause the upload for one day.

• On your iPhone, iPad, or iPod touch, open the Photos app, go to the Library tab, and tap All Photos. Then scroll to the bottom of your screen.
• On your Mac, open the Photos app. Select Library in the sidebar, then click All Photos in the list of tabs in the toolbar. Scroll to the bottom of your photos and click Pause.