How to enables my face id

1. Open the "Settings" app on your iPhone.
2. Scroll down and tap on "Face ID & Passcode" (or "Touch ID & Passcode" on older models).
3. You may need to enter your passcode to access the Face ID settings.
4. Tap on "Enroll Face" or "Set Up Face ID."
5. Position your face within the frame provided and follow the on-screen instructions to complete the scan. You will be prompted to move your face in different angles for better accuracy.
6. Once the scan is complete, you can choose to enable Face ID for various features such as Unlock iPhone, Apple Pay, or iTunes & App Store by toggling the respective options.
7. After configuring the desired settings, Face ID will be enabled on your iPhone.

Are you getting a message that your FaceID has been disabled on your iPhone? If so, you'll most likely need to contact Apple.

If an alert says Face ID has been disabled on your iPhone or iPad Pro - Apple Support

Genius Bar Reservation and Apple Support Options - Apple