I can't update my Mac to El Capitan
It seems somehow version of OS X on your mac is corrupted. Try restoring it using utilities in OS X recovery system (built-in).  

To start up from OS X Recovery, hold down Command (⌘)-R immediately after turning on or restarting your Mac. Release when you see the Apple logo. If you can't start up from OS X Recovery, try holding down Option-Command-R to start up from OS X Internet Recovery. Startup is complete when you see the OS X Utilities window. Use one of the listed utilities to restore your mac.  


If you have time machine backup, select Restore From Time Machine Backup. 
Otherwise, select Reinstall OS X. OS X Recovery (Command-R) installs the version of OS X that was last installed on your Mac. OS X Internet Recovery (Option-Command-R) installs the version that originally came with your Mac.
If still not able to restore, browse the web and find help for your mac using safari (Get Online Help Option).