I have Mi A1 mobile.
1. Select front camera
2. Tap finger print sensor
Selfi is not taken.
Android version: 7.1.2
Security patch level: September 1 2017
Seems that feature was on MIUI. Your phone runs of stock Android so it's not there. What you can do is use audio to command the phone to take photos.
Are you on the best mobile phone tariff? Use our free comparison calculator to find out.