Error Message :
U02000191 No valid command is specified with RunID '1107005'.
Using a VARA.MULTI (union/difference/intersection) causes the above error on usage or preview, where the sources are 2 dynamic backend variables.
The single backend VARAs work and can be previewed:
As a workaround, VARA.STATIC can be used, by taking the result from the VARA.BACKEND and writing it to a VARA.STATIC before passing to the VARA.MULTI: