Also which method are you using? Have you tried it both ways?
Are you using the Windows Context Menu by selecting the files in Windows then right click, "Combine Files in Revu"?
Or do you have both files open in Revu then File>"Combine Files">"Add Open Files"?
If those both fail you can try going to C:\Program Files\Bluebeam Software\Bluebeam Revu\2018\Revu and then double click the file called Stapler.exe and then retry it.