So tried upgrading to El Capitan last year on my macbook and it failed. Things about mac is once they fail trying to upgrade its a bitch to get them back up. It made me format and go back to lion, cause I hadnt had my macbook up to purchase mavericks or el capitan or high sierra, Mountain Lion is not available to me as well, so it used my default one for my macbook pro, Lion.
So the error message you get when you try to upgrade your mac has to do with the time on it, since i dont have a battery in my macbook i need to add a ntpdate time.apple.com to my start up. This will actually avoid a lot of issues.