Online check-in may be convenient for both travelers and airlines, but it isn’t available for every traveler on every itinerary. Following are a few of the reasons you might be forced to check in at the airport instead of online:

Travelers traveling with pets.
Travelers traveling in a group of 10 or more persons.
Any traveler who is an unaccompanied minor.
Travelers traveling with a lap infant.
Travelers who are traveling with military Id instead of a passport.
Travelers who are non-US citizens departing from international locations.
Travelers who purchased an extra seat or special items.

Note: If you are not any kind of traveler as mentioned above but still can’t do web check-in then feel free to contact the Spirit Airlines customer service.
To get more info visit : https://spritfly.com/blog/spir....it-airlines-web-chec