The error you’re seeing when you add a www prefix, or any prefix, to your myshopify URL is expected. It happens by design and is intentional due to the nature of our security certificates. This doesn’t mean your domain has security flaws, however, it means you’re only able to use themloft.myshopify.com.
If you wish to add a www prefix to your domain, I suggest buying a custom domain. When you buy one you get two versions of your domain by default–one with the www prefix and one without. I don’t only suggest buying one for this reason, but also so you can match your domain with your brand. You can buy a custom domain such as themloft.com. Doing so can help you build trust in your online store, and it can bring several other benefits to your business. To buy a custom domain, you can refer to the guide on this page.