Unlock MacOs with fingertips doesn't work on Big Sur
This has recently happened to my sister's Macbook Air 2019. The solution for me was resetting the SMC. To do so in a T2 equipped Mac device:

Shutdown your Macbook.
Press and hold the power button for 10 seconds.
Wait a few seconds and power on your macbook again.

Reference: https://support.apple.com/en-ca/HT201295