Where are the invoice settings?

Go to Appearance -> Themes -> Add New -> Upload Theme -> Browse and Install Now You will need to install some required plugins to work the theme properly. Follow the below screenshot.

To create google API key visit the link https://console.cloud.google.com/ and login with your google access. After that, you need to create a project for API credentials with some Library permission. Follow the screenshot below: Library Support: Key Restrictions:

Restrofood provide invoice logo, shop name, address and footer text add option.

To work time and date properly with your time you need to settings restrofood plugin timezone from restrofood settings and WP admin settings -> general Restrofood settings option: WP admin settings option:

To create product attributes follow the screenshot: Set Product Variations: