I've never traveled with Spirit Airlines but usually you have various alternatives if online checkin doesn't work (which happened to me some times):
- Check in at check-in desks. Especially if you have a baggage that needs to be checked in, you have to go through this process anyway, so no time lost.
- use the automatic kiosk to print it.
In case you are traveling with a low-cost airline which requires to have the boarding pass already in your pockets when you arrive at the airport, or for any reason you want to arrive at the airport already checked-in, you have other 2 alternatives, but both requires a smartphone:
- download the airline app, if it exists. In this way you can check in from the app and having your boarding pass on the phone. no paper is needed, everything is done from the app.
- when you are redirected to printing page, use a pdf virtual printer (for example pdf creator) to have the boarding pass saved as pdf. Then you transfer it to your phone and that's it. There is a possibility that someone will not like your pdf boarding pass on the phone, but once you have it you should be safe. In the worst case, i.e. phone lost, battery dead, etc., the airline staff can find you in the system via your name.
That being said, I always use the app.
In case you travel with a big airline, the airline staff at the airport will be happy to help you printing a paper version of the boarding pass.