Error: "Variable "entry" does not exist" on basic template I'm new to Craft and I'm stumbling at the first hurdle for Craft 3

I'm getting "Variable "entry" does not exist." and I'm certain it's something so simple I'm missing.

Case:


Install craft 3 via composer 
Add a Single section called "Homepage" (URI: "/" | template: "index")
Add "templates/index.twig" - test it with basic markup; displays correctly
Add plain text field to homepage section ("homepageHeader")
Fill out homepageHeader in control panel and save.
Change template to literally only "{{entry.homepageHeader}}"
Error above after page refresh


I must be missing something VERY simple. What is it?
In order to set an entry as your "homepage" entry you have to leave the uri in the section completely blank. It does not work when you insert a / because this route does not fetch the correct element thus Craft is not able to provide the entry variable in your twig template