If you do not see Add to Schedule and Check-in button on a Session, it could be due to one of the two reasons:

  1. The Date & Time is in the past. (This reason is cancelled out in your case since your session in question is in future).
  2. The Session is set to be accessible for Check-in and Saving to schedule for custom roles. (Custom roles are roles with varied access, and are assigned to contacts. If you do not plan to set this session to be accessible by contacts with particular roles, then you can just reset the setting for this session to All usertypes by editing it.)