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.
It looks like the region was changed and, unfortunately, sometimes after the region change, pipelines will continuously fail (Microsoft claims this occurs because the region change is a manual process on their side and sometimes certain items, such as pipelines, do not get transferred). This can be resolved by simply removing and reinstalling the tenant.
If reinstalling didn't work, you will have to open a support ticket with Microsoft to resolve this issue (typically, when we have seen this in the past, uninstalling and reinstalling the tenant has worked; however, sometimes the only resolution for this region-change issue has been through a Microsoft support ticket). If you would like us to assist you with this issue, we will have to temporarily transfer ownership of the DevOps organization to us, as the owner is the only person allowed to open support tickets for the organization.
Please let us know if you would like us to open the support ticket and we will do so promptly.