mi a2 answer to phone

Not sure how Xiaomi phones work, but if just tapping the Answer button doesn't work, have you tried tapping/holding and swiping to answer? Do you need to unlock your phone first maybe?
 
Welcome to Android Central! You most likely have to slide that button. It might be up, sideways, or if the button appears within a dotted circle, slide it anywhere outside that circle.
 
I have the mi A2 also, on mine you swipe up to answer the phone call, swipe down to decline.