Unable to upload photos

If the photos and videos are appearing in an album "unable to upload" they are in a format, that is not supported by Photos.

Usually it helps to export these items, then remove them from Photos, and reimporting the exported items. This will convert the format to a more compatible format, if the system. still supports the old format. If Photos cannot reimport the items, try to find a third-party application that can handle the older format. It would have been easier to do this while still running macOS 10.14 Mojave. The older system versions supported more older formats.

See: Check the status of your library in Photos for macOS - Apple Support

"If you can't upload photos from your Mac

Unable to Upload [#] Items appears if there are any photos or videos on your Mac that couldn't be uploaded to iCloud Photos. You might be able to fix this with steps below, but edits and keywords that you applied to these photos will be lost.

1. Create a new Smart Album by choosing File > New Smart Album.
2. Name the Smart Album "Unable to Upload" and set the three pop-up menus to Photo - is - unable to upload to iCloud Photos.
3. 
4. 
   
5. Select "Unable to Upload" in the left sidebar.
6. Press Command-A to select all the photos in the Smart Album, and then choose File > Export > Export Unmodified Original For (#) Photos.
7. In the dialogs that follow, choose the file name format and location for the exported photos. Click Export, then click Export Originals.
8. After the photos export, make sure the photos are all still selected in the Smart Album. Then right-click or Control-click one of them, and choose Delete (#) Photos from the menu that appears.
9. Click Delete, then click OK.
10.  To import the photos back into your library, choose File > Import and find the images in the location you chose in step 5."