Q: Unable to select HDD when trying to upgrade

I am trying to upgrade to Mavericks from Snow Leopard, I have a MacBook Pro, Mid 2009 version with 2GB RAM. When I try and run the install and it gets to the part of selecting my hard drive, I try and select my main and only HDD, but it says "This disk cannot be used to start up your computer". I have tried to run it from an external hard drive as well, but the same problem occurs. What could be the problem?