Do you have a second Apple device using the same Apple ID and also syncing photos to iCloud?
If so, try AirDropping that image to this second device and see if it manages to unblock whatever's causing the issue.
Have you tried deleting the photo, then restoring it from the Recently Deleted album to see if it will sync then? (I don't know if the upload having failed interferes with this procedure)
Have you tried deleting the photo, then restoring it from the Recently Deleted album to see if it will sync then? (I don't know if the upload having failed interferes with this procedure)
Trick 2 did the trick ?Alright, I got two more tricks up my sleeve but then I'm out of ideas:
Trick 1:
Open the photo in Photos, tap on the Share icon, choose Duplicate
See if the duplicate syncs properly
Trick 2:
Note that if the original would be a Live Photo, the duplicate would no longer be a Live Photo with this trick
Open the photo in Photos, tap on the Share icon, choose Save to Files and save it somewhere you can find it
Open the Files app, find and open the photo, tap on the Share icon, choose Save Image
See if the duplicate syncs properly