rviz does not display Gazebo robot
I am trying to display in rviz my Gazebo robot following the book 'Programming Robots with ROS"code here I run the gazebo simulator Then I run rosrun rviz rviz and I get this
It is supposed that I should select "camera_depth_frame" from the Fixed Frame to the left, but you can see many red areas (errors) ("No transform")
Why is this happening and how can I correct it?