Why can't I see the Pokémon in a live camera mode?

You said in your comments that when you were asked if Pokémon Go could access your camera, you said no. This means that the game does not have permission to turn on your camera, so you won't see anything.

Granting Pokémon Go the permission to access your camera should fix this problem.

In iOS:

1. Open the Settings app
2. Scroll down to your list of apps and choose "Pokémon Go"
3. Switch "Camera" from off to on

In Android:

1. Open the Settings app
2. Tap “Apps” under the Device heading
3. Select Pokémon Go
4. Tap "Permissions"
5. Switch "Camera" from off to on

If you want to save any pictures you take, you should also turn on the ability to access photos.