Hello atia123,
For camera difficulties specific to FaceTime, you'll want to take the steps below:
• Quit FaceTime, then open FaceTime again.
• Restart your Mac.
• If you have multiple cameras or microphones connected to your Mac, check the Video menu in the FaceTime menu bar to make sure that you have the correct camera and microphone selected.
• If the issue involves your computer's built-in camera, learn how to resolve issues with the built-in camera.
You can also find those steps in this resource: If FaceTime isn't working on your Mac - Apple Support
Should that still persist, the additional steps below may also help: If the built-in camera isn’t working on your Mac - Apple Support
Update your software
Make sure your software is up to date.
Check Screen Time settings
If your Mac is using macOS Catalina or later and you use Screen Time, make sure your camera is turned on and apps that use the camera have time available.
1. Choose Apple menu > System Settings (or System Preferences), then click Screen Time.
2. Click Content & Privacy, then click App Restrictions (or Apps).
3. Make sure the checkbox for Camera is selected.
4. Click App Limits. If you need to use your camera with an app in the list, make sure the app is off or not selected.
If you are still unable to use your camera with FaceTime, you'll want to reach out to Apple Support: Apple Support They'll be in the best position to look into this further.
Take care.