OS X El Capitan installed on my IMAC

We are mixing upDate with upGrade. An update only happens within the existing version of macOS and in this case El Capitan. There have not be any updates for El Capitan in several years. As @ 6x6 has pointed out, an upGrade involves moving from one macOS like El Capitan to a newer version and just what this computer qualifies depends on the age of the machine.

Have a Read over Here and determine what this machine can move to. Before attempting, make a Time Machine Backup and remember, with UpGrades application compatibilities become a factor

It all depends non whether your Mac is "young enough:. Please see the requirements for macOS Sierra (which the the update from El Capitan): https://support.apple.com/kb/SP742?locale=en_US

Just Hope it all goes well.

Do come back if more issues arise or just to read others issues than can be used to fix yours.