Q: Why can't I install Mavericks on OS X Mountain Lion? I keep getting the message "Cannot install on Macintosh HD"

What model Mac Pro do you have?  Mountain Lion and Mavericks require a Mac Pro3,1 or better.

You have a Macbook Pro, not a desktop Mac Pro and, since yours is a 2011, it should install.

How much RAM do you have?

And how much empty hard drive space?

Did you get the machine new or did you buy it used with Lion installed?

That should install Mavericks, then.  Are you trying to install from the Mac App Store or the update download?  The correct place to go is the App Store to do the download so you are sure to have the full installer.

Is the installer in your Applications folder?

Make a copy of it some place safe like your Documents folder in case it gets messed up so ou don't have to download again.

What size is the installer file, should be round 5.3 GB.

By any chance do you have a BootCamp partition on this hard drive?

You might also try restarting on the Lion recovery hard drive by doing Restart and hold the Command and R keys.  Then from Utitilites choose Disk Utility and Repair Hard Drive so DU cleans up any damaged file structure on the startup disk.  Then restart normally and try again to do the insall.

Search your "Macintosh HD" for backup files "Backups.backupdb".  Mavericks won't install on a drive that contains time machine.....I read that somewhere.

What ssls6 i refering to is http://support.apple.com/kb/TS2986