Since Face ID uses the TrueDepth camera to authenticate your face, if something is obstructing or covering the front camera, it may not work. Smudges or gunk on the screen may impede Face ID from working properly, so clean your iPhone and try again.
Certain types of screen protectors or cases might obstruct the camera, so you should eliminate them and check if this fixes the issue.
Face ID won’t work if your nose and mouth are blocked, unless you’ve set up Face ID with a Mask. If you have set up this feature, you may need to hold the iPhone a little higher than you’re used to. It’s also worth making sure that nothing is blocking your eyes when you’re wearing a mask. To verify that the issue isn’t caused by facial coverings, eliminate any masks and sunglasses to check whether Face ID works.
If you have an older iPhone, Face ID will only work in portrait orientation. Only the iPhone 13 and later maintain Face ID in landscape mode, and even this doesn’t work if you’re wearing a mask.