OS X El Capitan No packages were eligible to install...

Hello,

I am having trouble with the OS X El Capitan update. It is a 21" iMac with Intel Core 2 Duo,3.06 GHz and 4GB RAM. I was using OS X Yosemite with no problems before attempting to update to El Capitan. I have never had an issue with any of the previous OS updates until this one and I was foolish enough not to back up my HD on Time Machine unfortunately. I installed the update but after it finishes, it prompts me with a screen that says:

OS X could not be installed on your computer

No packages were eligible for install. Contact the software manufacturer for assistance.

Quit the installer to restart your computer and try again.

Would anyone know what to do with this issue? I have tried at least 5 times to reinstall. I also tried to access my files on my HD which still show that it is there but I could not drag it over to an external 1TB HD.

Any help would be greatly appreciated.

Thank you in advance!