App Store not allowing me to upgrade to Sierra
Forget that guide. It is incorrect at this point and giving you misleading instructions.

The Update tab is only for updates for your currently running system.

As you phrased correctly in your question title, you want an upgrade.

That means a completely new system.

Currently, the Sierra installer – or upgrade – is on the first tab. The Featured-tab marked with a star. Top link on the right. Text only.

You may also search for "macOS Sierra" in the search box, upper right corner.

All respecting the prerequisite that your machine is supported for this kind of upgrade.