How to rollback macOS Sierra Developer Beta back to El Capitan (without time machine)
macOS is not designed to be downgradable, and as such you cannot downgrade it through an installer like this. You will need to create a bootable install disk (Apple support), e.g. by running 

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


to install on a drive called MyVolume. Running this will lose you all your content, so you will need to have a backup if you want to retain your files. Otherwise, wait Sierra out…