1
Open the Settings App.
Go to Privacy & Security - Camera
In there, is the Webcam enabled, and if so, in the list of apps, is the app that is producing that notification enabled to allow it to access your webcam?
2
If those settings are enabled and you still cannot use the webcam, open the built-in Camera app in Windows.
In that app, is your webcam working, or is there an error message, or camera icon with a line through it?
If so:
Look at the function keys (F1 - F12) on your keyboard, if one of them has a camera icon, press that key or Fn + that key to unlock the webcam.
If that was not the problem, look at the camera lens above your screen, is there a small privacy slider that needs to be opened to access the webcam?