Using El Capitan 10.11.6 can not update to macOS Sierra 10.12.6

This is what is known as a Catch 22.

Unfortunately, your iMac cannot be updated to run OS X Sierra, because it does not meet the requirements to do so:

MacOS Sierra - Technical Specifications

Yet, in order to update Pages, Numbers, and KeyNote, you need to have OS X Sierra installed on your Mac.....which cannot be installed on your Mac.

About all that you can do is simply ignore the messages to update Pages, Numbers and KeyNote when they appear.

Still likely caught in a Catch 22, I am afraid.

Reason.....if you look at Pages, Numbers, Keynote, etc in the App Store, they now all list a requirement of Sierra 10.12.x for installation.

You could have installed Pages, Numbers, and KeyNote when they were originally offered in El Capitan 10.11.x, but I doubt that you can do this now, since it has been some time since Sierra was introduced.

Whether or not you will see a different requirement when you check the App Store with El Capitan 10.11.6, I cannot say.

Ok so your mac won't upgrade to macOS Sierra, bad karma on Tim and his billionaire cronies.

So you are stuck with El Capitan on a very capable machine, but yet you are not allowed to update apps,

unless you add to their burgeoning wallets, they could have made provision to support those you cannot afford to buy the latest macs, but they didn't.

However a bit of creative googling will give you suggestions as how to upgrade your unsupported mac to macOS Sierra.