Unable to install El Capitan

I have downloaded OS X El Capitan and bring to install to a SSD which has been formatted as Mac OS Extended (Journaled) - GUID. (Trying to install on iMAC 2008 model.)

When I try to install to it, it says "OS X can't be installed on this isk. macOS isn't installed"

I am unable to get into Utilities on boot up as the recover partition is not available nor can I do recovery using internet. I have tried repairing the recovery partition but not been successful. Hence why I am trying to install manually.

Any help is much appreciated.

Thanks