roslaunch throws RLException

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Hello, I am trying to follow this tutorial:

http://wiki.ros.org/ROS/Tutorials/Usi...

I followed all instruction in the tutorial, including installing the requested packages, until section 2.5

Then, when I type the following on the comand line,

roslaunch beginner_tutorials turtlemimic.launch

I get the following error:

RLException: [turtlemimic.launch] is neither a launch file in package [beginner_tutorials] nor is [beginner_tutorials] a launch file name
The traceback for the exception was written to the log file

I have no clue what is going on here. Could anyone help me in fixing it?

my distribution is:

ROS_ETC_DIR=/opt/ros/melodic/etc/ros
ROS_ROOT=/opt/ros/melodic/share/ros
ROS_MASTER_URI=http://localhost:11311
ROS_VERSION=1
ROS_PYTHON_VERSION=2
ROS_PACKAGE_PATH=/home/giuseppe/catkin_ws/src:/opt/ros/melodic/share
ROSLISP_PACKAGE_DIRECTORIES=/home/giuseppe/catkin_ws/devel/share/common-lisp
ROS_DISTRO=melodic

I thank you in advance. My best regards, Giuseppe