WQA: The yellow error message appears at the top. I know this makes no sense here...baffled
Actually this does make sense since this process will first try to save the customer record before adding a document to their name. It sounds like you have some duplicate customer names in Method which you'll need to clear up. I would first do a full sync to make sure Method and QuickBooks Online data are in sync, click the QuickBooks > Synchronize tab then click the first Sync now... button. Wait for that to finish then review the Resolve Conflicts tab link to see what's showing - I think you're going to see some conflcits in here on duplicate customer names. If you do have duplicates make sure you delete from Method.
Once that's all cleared up try to attach the document again and let me know how it goes.
~C