You should be able to check-in online, but sometimes you are not able to generate a BP in the app or on the website. Then you need to approach the check-in counters or a check-in kiosk at the airport to get your BP.

My DP is a recent Pegasus flight BLL-AYT where I was able to check-in in the app but only able to get a BP in the airport. For subsequent Pegasus flight ex. AYT and SAW I was able to generate BP in the app.