Hi,
This error means one of your backup operations (from Dec 20th at 01:00) has not marked as completed.
You can try to cancel it manually. Open a browser and navigate to https://{your_vcsa_address}/apiexplorer/. Click on the Login link at the top right of the screen and provide your admin credentials (eg. administrator@vsphere.local / secretpass). In the Select API dropdown at the top center, select 'appliance'. In the listed APIs, search for groups labelled recovery/backup/job (backup operations) or recovery/restore/job (restore operations). Expanding the corresponding group, you should be able to get access to the relevant job APIs, like getting a list of current jobs and cancelling a job.
Once you got there, try to find your stuck job (with ID 20181220-010007-9451637), and you can try to get its status and to cancel it via the corresponding API calls.
Another option, possible easier, is to use VCSA Backup UI at https://{your_vcsa_address}:5480/ui/backup