The Flex theme is sold outside of the Theme Store, so adding the theme to your Shopify store requires a different approach.
Download the zip file from the purchase email
When you receive your purchase email, click the link to download Flex.
The theme will download as a ZIP file called unzip-me-first.zip. You'll need to unzip this file, which creates a folder called Welcome to Flex.
Open the "Welcome to Flex" folder
There are 14 ZIP files in the Welcome to Flex folder.
There will be a main flex.zip file, as well as zip files named after the current 13 demo shops (ex: flex-swagger.zip).
Upload your preferred preset
Open your Shopify admin and navigate to Online Store, then Themes.
Under More themes, click the Upload theme button.
Click Choose File and select the ZIP file for the preset, then Upload.