A couple of things you may want to try.
Look in your Applications folder for Install Mac OS X El Capitan.app.
If you have this move it to the trash and delete.
Also look in Launchpad for an icon with the OS X logo, if you have it
click and hold till it wiggles, click the 'x' and delete.
Restart the mac and relaunch the App Store, try and see if it will download now.
also if you are trying to download via WIFI try connecting by ethernet if you can.
If these do not work you can try removing another folder from the /private/var/folders/zz path

Above is the path to the folder that needs to be removed.
The folder is normally hidden by default, to make it visible follow instructions below.
In Finder use the Go To Folder item under Go and type in /private/var/folders in the text box
and from there navigate to zz and then the folder zyxvp........ ending in 68, control-click and then Move To Trash, don't delete
you could also drag it to the desktop before for extra safety.
Restart the Mac and relaunch the App Store and see if you are able to download now. The folder and files within it are regenerated on Restart.
If you are happy you can then empty the Trash and delete the one on the desktop if you did that.
If you are successful and the download completes, do this before you install it.
Look in your Applications folder for Install Mac Os X El Capitan.app, make a copy of it on an external disk or USB device.
This will save you having to download it again in case you need to.