If by this
"I am getting an error with the mount.bat file..."
you mean that you are getting a Mount REquest, don't even consider what media is being requested. The Mount request means that there is no writable media in the media pool you are using for backup
Go to the Destination tab of your Backup specification, get Properties for the device(s) that you are using, and make note of what media pool is being used
Go to Devices and media, expand your library, single-click on the word 'Slots' and sort the media by Pool by clicking the column header
For the media in the Media Pool you want to use, see if you have media whose Protection is set to either None or Expired. Check the Condition field on the far left to make sure that thsi media is Green