You must first enable the Checkout feature under Configuration > General to access this functionality. You can enable anonymous checkout mode to speed up the checkout process for customers by selecting the Enable anonymous checkout option from the Configuration > Checkout menu. The Storefront will automatically create a new guest account for the anonymous shopper upon placing order bypassing the standard login and registration forms in a normal checkout process. In anonymous mode, the customer will not be able to login to their newly created account unless you explicitly provide the login and password to the customer.
It's important to understand the Storefront creates a new account with the customer's email address captured during checkout. By default, your site allows multiple accounts with the same email address. If, however, you have explicitly enabled the Requires unique email address under Admin > Site Settings page, the Storefront will not be able to create the guest account if the email has already been used before. In such cases, you must also enable the Reuse anonymous account under the Configuration > Checkout settings.