OS X could not be installed inyour computer

I tried installing El Capitan on my uncle's MacBook pro 2011 but it's saying that OS X could not be installed and that no packages were eligible for install. I've tried everything on the internet but it just keeps on going the same process over and over and over again. It only gives me an option to restart or shut down and when it turns back on, it tries to install El Capitan again and ends up in the same error. It's a never-ending loop. I just want the normal screen back