Error:
TF400864: The Item specified by the key 12345678-1234-1234-1234-01234567890 does not exist.Resolution:
The Sync is failing because of a known bug with changing the Azure DevOps organization region.
After changing the region of your Azure DevOps organization, pipelines can continuously fail. Microsoft claims this occurs because the region change is a manual process on their side and sometimes certain items do not get fully migrated.
To resolve this issue, first attempt to remove and reinstall your tenant. If reinstalling does not work, you will need to open a support ticket with Microsoft to resolve this issue. Microsoft support will require some information regarding your Azure DevOps organization and should be able to restore Azure Pipeline functionality.