Unlock MacOs with fingertips doesn't work on Big Sur

I'm trying to unlock my Mac with Touch ID. During the initialisation process, I configured my fingertip correctly but when I try to unlock my Mac it doesn't detect it.

If I go in "System Preferences" under the menu "Touch ID" this is what I see:

Then I decided to check the "Unlocking your Mac" option, I fill the password but if I open "Touch ID" for the second time "Unlocking your Mac" is still unchecked.

Why? What can I do?

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