Unable to create or edit Relationships between published data sources.
The ability to define/edit relationship between published data sources is not a feature built in the product as described in the Tableau Help documents linked below:
Relate Your Data: Requirements for relationships
Relate Your Data: Data that can't be related
The data source relationship needs to be edited before publishing.
To voice your support for the future implement of this feature, please vote on the following idea:
Enable relationships on published datasets
To confirm the relationships, you need to open the edit page of the data source, and since a published data source is already considered "completed and shared", you cannot check it directly on the Tableau Server-side currently. The following knowledge base contains the relevant specifications.
Editing a Published Data Source On Tableau Server
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