Are you saying that is the error you get when you try to download? Or have you downloaded and that is the error you get when you try to open the downloaded file?
It sounds like the .dmg file is corrupt and can't be mounted. If that is the case you need to download it again.
Have you downloaded and successfully installed other apps? If so, it may be the permissions on the dmg file need repairing.
If it happens for all dmg files, make sure your OS is fully up to date.
I just got it two days ago so I can't see why it wouldn't be. I downloaded a couple files fine yesterday but I tried another file just now and that one didn't work either. Grr. Thank you.
There's no telling how long ago the machine was imaged even if you did buy it recently.
I would say making sure all the latest system updates are installed is your next step.