my Mac is mid 2011, 21.5 inch (not listed here) - 10.12.6. Can I upgrade?

You can upgrade a mid-2011 iMac to High Sierra 10.13.6

See instructions in How to get old versions of macOS - Apple Support

And see the following from Identify your iMac model - Apple Support

iMac (21.5-inch, Mid 2011)

Model Identifier: iMac12,1

Part Number: MC309xx/A, MC812xx/A

Newest compatible operating system: macOS High Sierra 10.13.6

Tech Specs: iMac (21.5-inch, Mid 2011)

User Guide: iMac (21.5-inch, Mid 2011)

Yes your mid 2011 iMac can upgrade to macOS High Sierra 10.13.6, but that is all.

Open Safari and click on this link, 

How to get old versions of macOS – Apple Support

Go to the Download OS section and click on macOS High Sierra.

Before upgrading make a full back up of your Mac.

Check that third party apps you use have updates available for the new OS.

https://roaringapps.com

Check that your printers/ scanners etc have updated drivers and software available for the new OS.

Connect your mac to your router via cable rather than rely on WiFi.

Do not use your mac while it is downloading

and do not use any other devices that share the same internet connection.

If the App Store says ‘Open’ then it is detecting a previous download of the install app

go to the Applications folder locate Install macOS High Sierra.app.

Move it to the Trash, empty the Trash.

Restart, click on the link again, the App Store should say ‘Get' or 'Download'.