Creating a child theme – This dummy needs help please

Right,

I have looked at both video tutorials about creating my own child theme and I understand the necessity of doing so in order that future updates of the core theme won’t break my DirectoryPress website – so far so good.

So I go to the create a child theme page and tick all three boxes and can download a Zip folder.
Upload folder to Appearance >Themes >Add Theme >Upload and so on and I get the following

Unpacking the package…
Installing the theme…
This theme requires a parent theme. Checking if it is installed…
Preparing to install …
Downloading install package from https://www.premiumpress.com/_childthemes/download.php?key=%5BXXX%5D…
Unpacking the package…
Installing the theme…
The package could not be installed. The theme contains no files.
Theme install failed.
Theme installed successfully.

I return to the Themes page and get

Broken Themes

The following themes are installed but incomplete. Themes must have a stylesheet and a template.
Name Description
Water Gardening Directory – Child Theme 1 The parent theme is missing. Please install the “[XXX]” parent theme.

So, I have a look at the files i have uploaded (checked on my computer not in WordPress folders) – I have
functions.php
style.css
index.html
and an image file

All appears to be present and correct although i am surprised to find the index.html file is entirely empty – can this be correct? If so why? And does that mean I have to rebuild the homepage of the website again? Academic question at the moment since the child theme is broken anyway!

Hugely out of my depth here – any help gratefully received. Many thanks
Patrick