This disk can not be used to start up your computer

I am running snow leapord and need to upgrade to a newer OSX. I get the same message when I tried to install either OSX El XCapitan or OSX Yosemite, I get the same message "This disk can not be used to start up your computer." I have 13GB free on my 128 GB Hard drive. I'd rather not back up and erase my HD. There are no partitions on my hard drive. I've used disk utility to repair any permissions. Any thoughts much appreciated...