Every time I try I get this message:

What happened?
A system error occurred while trying to process your request to check in for your flight.

What you need to do:
Please use your browser's back button to return to the previous page and submit your request again. You may also check in at the departure airport at a Southwest Airlines Skycap Podium, Ticket Counter, or RAPID CHECKIN kiosk to obtain Boarding Pass or Security Document before proceeding to the security checkpoint.