https://www.southwest.com/getbusinessselect/index.html
It automatically checks you in at 36 hours in advanced though you will need to go through the check in process at 24 hours in advance in order to get your actual boarding pass. The system will assign you a boarding position anywhere between A1-A15 which
should depend on when you purchased your ticket.
The reason you were not able to purchase it when you bought your tickets is EBCI isn't an option when you purchase Business Select because you're already getting the automatic check in (Same for A-listers, it's not an option).
https://www.southwest.com/help/booking/earlybird-checkin
Customers who have purchased electronic tickets at the Wanna Get Away® or Anytime fares can benefit from purchasing EarlyBird Check-In®. Priority boarding privileges are already included with the purchase of Business Select® fares and are a benefit of being a Rapid Rewards® A-List Preferred or A-List Member or a Passenger on a reservation that includes an A-List Preferred or A-List Member.
EarlyBird Check-In® Customers will receive boarding positions after Business Select Customers and A-List Preferred, as well as A-List Members and Passengers on the same reservations as A-List Preferred and A-List Members.
NOTE: EarlyBird Check-In® should not be purchased for (1) for Passengers on the same reservation as an A-List or an A-List Preferred Member as they will receive priority boarding or (2) Unaccompanied Minors since they will preboard the flight. Please review our unaccompanied minor policies for more details.