Well this could mean your download of Mojave is damaged in some way so may need to be
deleted and downloaded again.
But first boot into Safe Mode and see if you can install Mojave. Use safe mode to isolate issues with your Mac - Apple Support
Or there is something not quite right with your install of El Capitan, open Disk Utility and
run First Aid to check for any errors, if any errors are found boot to your Recovery HD
open Disk Utility and run First Aid until you get no errors.
Then try to install Mojave again.
If that does not work download and install this, https://support.apple.com/kb/DL1885?viewlocale=en_US&locale=en_US
it's fine to install this over what you have it won't affect your files but will reset some system files and reset caches.
now try installing Mojave.
Some might say try installing High Sierra first, How to upgrade to macOS High Sierra - Apple Support
although it is not necessary.