Hi Ron
Passing over the difficulty of getting Sierra for the moment -
The process is:
First download the latest version of Diskmaker X from here
http://diskmakerx.com. There is a Sierra-compatible version.
Make sure you have a good-quality USB Stick of at least 8GB.
Download Sierra from the App Store. When it finishes the download, it will be located in the Applications Folder. Do not initiate the installation process.
Instead, fire up Diskmaker X. Follow the instructions. It will look for, and find, the Sierra download. It will format the USB Stick and create a Bootable USB Installer. This takes time. Be patient.
When it is finished, you have various choices. You can go into Applications, click on the Sierra download and initiate the installation. When that is finished, the Sierra download folder will self-destruct. But you still have the Diskmaker Bootable Installer USB Stick (a) as a backup in case you ever need to re-install Sierra or (b) use it to install Sierra on another Mac.
Or, you could copy the Sierra download folder to an EHD as a copy - just in case. Then use the original to install Sierra.
Or you could stop the whole process there. And keep the Sierra download Folder AND the Diskmaker Bootable USB Stick for another time.
To use the Bootable Installer USB at any time in the future, reboot the Mac whilst holding down the Option key. Then choose that Stick to install Sierra.
That's a brief account. Masses on the Internet about how to do all that.
Now - why doesn't Sierra show up in the App Store under purchases? Obviously, you have to have downloaded it at sometime in the past with the same Apple ID. If you haven't, then Sierra should appear as an upgrade if your Mac is compatible.
Ian