Correction: the button is in the UPPER LEFT corner, under the mountain image.
Go to:
Scroll down to where it says: "If you still need Sierra, follow these steps:"
Follow the link. Connect to the App Store. Click on the button in the lower left, under the mountain image, that says "Get" and it will give you another window for the license and the installation. (Don't look for a "Download" button as there isn't a "Download" button. Don't check out the downward triangle for the drop-down menu; you don't need that, either, and it won't take you to installation. Just click on "Get".)