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