Ubuntu 18.04: ros-melodic-turtlebot3-rviz-launchers package is not being located

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Your question is not very clear, but I guess you are trying to run Turtlebot 3 in Melodic. This is a simple guide that I made to myself to install the TurtleBot3 packages and run it using RViz and Gazebo. I only tested it in simulation because I don't have access to a Turtlebot3. The information here is a mix of the content of chapter 10 of the book [1] with a couple of adaptations based on tests I made on ROS Melodic running on Ubuntu 18.04. I hope this helps.

Ref.: [1] Pyo, YoonSeok, HanCheol Cho, R. Jung, and T. Lim. "Ros robot programming." ROBOTIS, December (2017).

Installation

To install Turtlebot3 on ROS Melodic (for simulation), execute the following steps:

Install dependencies:

$ sudo apt-get install ros-melodic-joy ros-melodic-teleop-twist-joy ros-melodic-teleop-twist-keyboard ros-melodic-laser-proc ros-melodic-rgbd-launch ros-melodic-depthimage-to-laserscan ros-melodic-rosserial-arduino ros-melodic-rosserial-python ros-melodic-rosserial-server ros-melodic-rosserial-client ros-melodic-rosserial-msgs ros-melodic-amcl ros-melodic-map-server ros-melodic-move-base ros-melodic-urdf ros-melodic-xacro ros-melodic-compressed-image-transport ros-melodic-rqt-image-view ros-melodic-gmapping ros-melodic-navigation ros-melodic-interactive-markers

Download the Turtlebot3 packages and build them:

$ cd ~/catkin_ws/src/
$ git clone https://github.com/ROBOTIS-GIT/turtlebot3.git
$ git clone https://github.com/ROBOTIS-GIT/turtlebot3_msgs.git
$ git clone https://github.com/ROBOTIS-GIT/turtlebot3_simulations.git
$ cd ~/catkin_ws && catkin_make

TurtleBot3 Simulation using RViz

To launch the virtual robot, first create the model variable in ~./bashrc file. Possible models are: 'burger', ‘waffle’ or ‘waffle_pi’:

$ export TURTLEBOT3_MODEL=burger

Then, run the launch file to load the model of the Turtlebot in RViz, execute the turtlebot3_fake_node (which publishes "fake" topics), and run the robot_state_publisher node (which publishes the transformation of each wheel to the topic /tf):

$ roslaunch turtlebot3_fake turtlebot3_fake.launch

You can run the teleop_key launch file to operate the robot using your keyboard. Open a new terminal window and run:

$ export TURTLEBOT3_MODEL=burger
$ roslaunch turtlebot3_teleop turtlebot3_teleop_key.launch

TurtleBot3 Simulation using Gazebo

Run the launch file that loads Gazebo, the environment and the robot model (I am using a different model here because I wanted to see the camera image in RViz):

$ export TURTLEBOT3_MODEL=waffle
$ roslaunch turtlebot3_gazebo turtlebot3_world.launch

To move the robot with the keyboard. Open a new terminal and run:

$ export TURTLEBOT3_MODEL=waffle
$ roslaunch turtlebot3_teleop turtlebot3_teleop_key.launch

Instead, you can run the next launch file that will make the robot navigate randomly, avoiding obstacles:

$ roslaunch turtlebot3_gazebo turtlebot3_simulation.launch

RViz can visualize the position of robot, distance sensor data and camera image while the simulation is running on Gazebo. To see this, open a new terminal and run:

$ export TURTLEBOT3_MODEL=waffle
$ roslaunch turtlebot3_gazebo turtlebot3_gazebo_rviz.launch

RViz will show you the robot and the point cloud of its LASER sensor. You can also visualize the image from the robot camera by enabling the Camera in the "Displays" window. After that, expand the Camera menu, go to Image Topic and change it to /camera/depth/image_raw to visualize the depth image.