For anyone having this problem on their PS4 preventing them from posting screenshots to social media. What this means is that your upload notification queue is full. Most likely with redundant notification that a save-game could not be uploaded because Sony seriously dropped the ball on the whole cloud save storage this generation.
Yes, that's pretty counter intuitive and will probably be fixed at some point.
Just go to your notifications, uploads, select the first notification and hit the option key to delete it. It's a tedious process but it'll work.