For Drupal Commerce on Drupal 8, Rules is not complete or integrated. This leaves many such tasks like this for custom development. Fortunately, it's also easier to alter individual parts of the Commerce system thanks to Drupal 8's object oriented architecture. In this case, you'd write a new class called an event subscriber to assign that user role on the "placed" transition - the point at time an order completes checkout or is placed by an administrator. It should then handle assigning the user role to the order owner.
There may be contributed modules available for Rules integration to support individual tasks like this, but it hasn't been a core priority as of yet while we focus on other major feature groups (e.g. shipping, addressbook, recurring billing, etc.).