Assigning a membership to a user requires that we have a user to assign that access to. Guest checkout should be disabled in your WooCommerce settings.
To disable guest checkout, go to WooCommerce > Settings and select the Accounts & Privacy tab. Uncheck “Allow customers to place orders without an account.”
Note that the other Guest Checkout and Account Creation settings are optional. However, allowing guests to log in during checkout and allowing customers to create an account during checkout do smooth out the user experience.