How do I get the “Run as Admin” choice?

I followed the steps provided, but when I right-clicked, I was no “run as administrator” option to click. When I right-click on other programs, however, that choice appears. I took ownership of the command prompt, but that changed nothing. I tried to run sfc/scannow, but I was told that I needed admin privileges to do that. I am the only user of this desktop PC and my account clearly says that I am an administrator. Why am I locked out and how can I change this?