Here are some steps you can try to fix the problem with Face ID:
Restart your iPhone: Press and hold the Side button and either volume button until the slider appears. Drag the slider to turn your device completely off, then press and hold the Side button again until you see the Apple logo.
Check Face ID Settings: On your iPhone, go to Settings > Face ID & Passcode and make sure that the Face ID feature is turned on for iPhone Unlock, iTunes & App Store, and other apps that use Face ID.
Reset Face ID: On your iPhone, go to Settings > Face ID & Passcode and select Reset Face ID. Follow the instructions to set up Face ID again.
Check for Software Updates: Check for any available software updates, as the latest update may address the issue with Face ID. Go to Settings > General > Software Update.
If none of these steps work, you may want to contact Apple Support or visit an Apple Store for further assistance.