Cannot make a bootable USB drive for El Capitan

I have upgraded my 2009 iMac to a lovely Late 2015 5K Retina machine and I have struggled my way through the migration from the old to the new. Now I am following the advice to erase the disk on the old machine so if someone wanted it, it would be out-of-the-box for them. I have fallen at the first hurdle as using Disk Utility from restart and Option R fails as the disk will not unmount. I have therefore tried to follow the next advice and make a bootable USB drive with OSX El Capitan and I am failing there too as when trying to create it, Terminal tells me 'createinstallmedia: command not found'. I am not terribly familiar with terminal command so have been relying on Apple Support but unfortunately it always assumes everything will work and mine doesn't.

I have downloaded the InstallMacOSX.dmg which is in Downloads and upon double clicking it I have a window with InstallMacOSX.pkg in it. I have tried the

sudo /Applications/Install\ OS\ X\ El\ Capitan.app/Contents/Resources/createinstallmedia --volume /Volumes/MyVolume --applicationpath /Applications/Install\ OS\ X\ El\ Capitan.app

command and this is when I learn that the command is not found. I am not convinced that I have the right things in the right places so any advice any of you Apple Geniuses can give me would be gratefully received. All I want to do is erase my disk and put my old Mac back in its box.