Mac OS X El Capitan not upgrading to Sierra 10.12

Hi, I have an iMac (21.5 inch Late 2013) that runs Mac OS X El Capitan 10.11.6. I am trying to upgrade to OS Sierra 10.12 from the Apple download page for older versions link. I have gone through link download and installation process twice and I am told the upgrade is complete. However, when I go to "About This Mac" it still says I am running El Capitan 10.11.6, even after restarting my computer. I have 152 G of free space on my computer so I don't think that is the problem. Is my computer no longer compatible for upgrades?