The calibration screen opens when you select Practice or Play for the first time from the song start screen. Calibrate your piano by playing the keys you see on screen. The key shows a check mark ✔︎, when Yousician hears you play it. The calibration also shows where your fingers should be when the songs starts, and it's time for you to play. You may also select Skip to start the song.
If Yousician can't hear you at all, check troubleshooting tips here.