It isn't uncommon for first time users to find calibration difficult. Rest assured it becomes easier every time you use the app.
To make the calibration process easier make sure your face and hands are as still as possible. Remove glasses or anything that might hide your face. Items that stabilise the phone, such as a stand, can be very helpful.
The app works on almost every smartphone launched since 2018, but if this issue continuous, please contact us for support.