External Users Face a data Error when using User Functions: DataServiceFailure User(s) do not have permission to access datasource.

This is considered a feature request:

From our goal to use RLS to limit access to DSSS data while still permitting edits, this is not possible and there’s an inherent architectural issue—you can’t allow users to edit a blended workbook and simultaneously restrict their access to the secondary data source.

The most practical next step is to provide Connect permissions on DSSS for the relevant users or groups, then test the setup.

However, if business requirements dictate that these users should not access the DSSS data source at all, then Web Editing this workbook isn’t possible under the current architecture. To resolve this, the workbook would need to be redesigned—for example, by moving the DSSS sheet to a separate workbook or applying RLS directly within the data source instead of relying on blending.

Additionally, check:

1. Verify whether affected users or groups can be granted Connect permission on the secondary published data source DSSS, and test Web Edit again after granting that access. 

2. Users intended to Web Edit without saving changes must have Explorer, Web Edit, and Connect on the DSSS data source. 

3. If business requirements do not allow granting Connect permission to the data source DSSS, redesign the workbook architecture. 

4. Supported options from the case include separating the external sheet into a different workbook or applying RLS directly within the data source DSSS instead of relying on blending. For reference, review

https://help.tableau.com/current/pro/desktop/en-us/multipleconnections_troubleshooting.htm

https://help.salesforce.com/s/articleView?id=001472854&type=1.