Apple requires a valid business email to list your brand in their marketplace. They do not accept gmail, aol, or other non-domain emails.
In this step you will create your business email that you will use to setup your Apple account.
There are two options to create your business email: create the email from your domain provider or use improvmx to forward emails from your website host to your gmail.
Here are how to do this with some common providers:
If you have a website domain but not a business email, you can use the free service improvmx.com to forward domain emails to gmail. You will need access to your domain name provider (e.g. Square/Weebly, Google Domains, Godaddy).
Follow this guide to configure your DNS with improvmx.com.
Once you have a valid business email (e.g. info@yourwebsite.com), continue to Apple developer company setup.