"Cannot blend aggregated data from the secondary data source with the data on this sheet because there are no linking fields use din the view"
I'm using 2 separate data sources connected to Oracle and published to the Tableau server. Each has a joining field with the same name and Tableau shows that this field has been used in the Edit Relationship dialog to join the data sources
Are both joining fields actually being displayed in the view you are building? That's what this error message is complaining about. Having them joined in the Edit Relationship dialog is only the first step - they also must both be displayed in the viz (or at least be on the Level of Detail shelf)