It’s likely Apple’s App Store copy of El Capitan has been removed if you can’t get it from the store without an error.
If you are able to get the download to complete it will give you an error as it will automatically try to run the OS installer and as your system (if its a different system than the one you are trying to install it to) you are using is newer than what the OS-X is able to support it is popping an error. After exiting the installer you should be able to find the download file if you can then you can still make the OS installer following this guide How to make a bootable OS X 10.11 El Capitan installer drive Otherwise you’ll need to find someone with it already.
Now we have a second problem! These older Core2Duo systems don’t support USB boot. You’ll need to either make a FireWire bootable drive or use a bootable CD disk to install El Capitan