By default guest checkout is enabled in Drupal Commerce. You can find the
existing checkout flows here admin/commerce/config/checkout-flows.
There is a default checkout flow available. Go to admin/commerce/config/checkout-flows/manage/default
and find this section:
After making changes click Update and Save the form.
If you disable the Allow guest checkout, anonymous users will be asked to login when they are going to checkout.
If you enable the Allow registration, a registration form will be presented during the checkout process like this:
