Error "Flow Failed" After Failing to Union Tableau Server Data Sources using Tableau Prep Builder

Issue

When trying to run output after performing a union on two Tableau Server Data Sources in Tableau Prep Builder 2019.3, the following error might occur:

"Flow Failed"

Additionally, the following alert might appear in the upper right hand corner of the Tableau Prep Builder screen (red triangle icon):

"Error
Something went wrong and we were unable to process output step:
Failure evaluating to database: Internal Error - An expected error occurred and the operation could not be completed. DataServiceFailure
Try running the flow again or review the log file for more details."

Environment

Tableau Prep Builder 2019.3.1 or later
Tableau Server 2019.2 and earlier
Tableau Data Engine

Resolution

Option 1

Upgrade Tableau Server to 2019.3 or a later version. Click here for downloads of current and previous versions of Tableau Server: Tableau Server Downloads and Release Notes. For more information on current releases, see Upgrade Tableau Server and Server Upgrade.

Option 2

Try one of the following workarounds:

Connect to the published data sources using Tableau Desktop and extract them (save the extracts on the local machine), then use the Connect to Tableau extract option in Tableau Prep Builder's Connect list to connect to the local extracts. Then, union them and run the flow with no errors.
Or, right-click on the Union step and select Preview in Tableau Desktop. The flow completes and Tableau Desktop opens with a connection to the data source "Flow1-node-Union….". This will give access to the data after the Union step. From here, attempt to publish the data source from Tableau Desktop.

Note: If the data still needs to be cleaned after the Union, after this data source is published, follow the steps in Workaround 1 to create a local copy of this data, and then connect to the local copy in Tableau Prep Builder. Then clean the data and run the output step as expected.

Cause

This error occurs because there is a version compatibility issue between Tableau Server (2019.2 and prior) and Tableau Prep Builder (2019.3 or later).

Performing a Union of Tableau Server Data Sources in Tableau Prep Builder 2019.3 and later will only work if the Published Data Sources are located on Tableau Server version 2019.3 or later, regardless of the underlying data types that were used to create the Published Data Sources.

Additional Information

For more information about the compatibility topic, see the section "Tableau Prep Builder all versions" in the Tableau Help documentation: Version Compatibility with Tableau Prep Builder.