You may not have permission to add files to that folder.
Try changing the backup location to your My Documents folder as a test and see if that works. If it does you either need to use a different directory so that you have permissions or amend the permissions on your Program Files (x86)\Tableau folders.
"However, as a TEST, if I am explicit about the filepath then it works as it should. This works:
tabadmin backup --date "C:\Program Files (x86)\Tableau\Tableau Server\backups\backup_QA"
As stated I do have permission. I use the same path for dropping the zipped log files without issue.
Brett from Tableau support got me a solution...a simple one, too. Some times ya just need a second set of eyes and a quick test...
The SET command just needed the final back slash removed. So it went from this:
SET sPath="C:\Program Files (x86)\Tableau\Tableau Server\backups\"
SET sPath="C:\Program Files (x86)\Tableau\Tableau Server\backups"
Then all I had to do was add the final back slash in my code. So from this:
tabadmin backup --date %sPath%backup_QA
tabadmin backup --date %sPath%\backup_QA