i am trying to go throught this tutorial

http://wiki.ros.org/tf/Tutorials/Writ...

but when i use

$ roslaunch learning_tf start_demo.launch

i am getting this error

[turtle1_tf_broadcaster-4] process has died [pid 12905, exit code 127, cmd /home/ros/catkin_ws/src/learning_tf/nodes/turtle_tf_broadcaster.py __name:=turtle1_tf_broadcaster __log:=/home/ros/.ros/log/81a985dc-2506-11eb-987b-d17fa1d300a7/turtle1_tf_broadcaster-4.log]. log file: /home/ros/.ros/log/81a985dc-2506-11eb-987b-d17fa1d300a7/turtle1_tf_broadcaster-4*.log

this is the complete thing

... logging to /home/ros/.ros/log/81a985dc-2506-11eb-987b-d17fa1d300a7/roslaunch-ros-12878.log Checking log directory for disk usage. This may take a while. Press Ctrl-C to interrupt Done checking log file disk usage. Usage is <1GB.

started roslaunch server http://ros:38657/

SUMMARY

PARAMETERS
 * /rosdistro: noetic
 * /rosversion: 1.15.8
 * /turtle1_tf_broadcaster/turtle: turtle1
 * /turtle2_tf_broadcaster/turtle: turtle2

NODES
  /
    listener (learning_tf/turtle_tf_listener.py)
    sim (turtlesim/turtlesim_node)
    teleop (turtlesim/turtle_teleop_key)
    turtle1_tf_broadcaster (learning_tf/turtle_tf_broadcaster.py)
    turtle2_tf_broadcaster (learning_tf/turtle_tf_broadcaster.py)

auto-starting new master
process[master]: started with pid [12886]
ROS_MASTER_URI=http://localhost:11311

setting /run_id to 81a985dc-2506-11eb-987b-d17fa1d300a7
process[rosout-1]: started with pid [12896]
started core service [/rosout]
process[sim-2]: started with pid [12899]
process[teleop-3]: started with pid [12900]
process[turtle1_tf_broadcaster-4]: started with pid [12905]
/usr/bin/env: ‘python’: No such file or directory
process[turtle2_tf_broadcaster-5]: started with pid [12906]
/usr/bin/env: ‘python’: No such file or directory
process[listener-6]: started with pid [12912]
/usr/bin/env: ‘python’: No such file or directory
Reading from keyboard
---------------------------
Use arrow keys to move the turtle. 'q' to quit.
[turtle1_tf_broadcaster-4] process has died [pid 12905, exit code 127, cmd /home/ros/catkin_ws/src/learning_tf/nodes/turtle_tf_broadcaster.py __name:=turtle1_tf_broadcaster __log:=/home/ros/.ros/log/81a985dc-2506-11eb-987b-d17fa1d300a7/turtle1_tf_broadcaster-4.log].
log file: /home/ros/.ros/log/81a985dc-2506-11eb-987b-d17fa1d300a7/turtle1_tf_broadcaster-4*.log
[turtle2_tf_broadcaster-5] process has died [pid 12906, exit code 127, cmd /home/ros/catkin_ws/src/learning_tf/nodes/turtle_tf_broadcaster.py __name:=turtle2_tf_broadcaster __log:=/home/ros/.ros/log/81a985dc-2506-11eb-987b-d17fa1d300a7/turtle2_tf_broadcaster-5.log].
log file: /home/ros/.ros/log/81a985dc-2506-11eb-987b-d17fa1d300a7/turtle2_tf_broadcaster-5*.log
[listener-6] process has died [pid 12912, exit code 127, cmd /home/ros/catkin_ws/src/learning_tf/nodes/turtle_tf_listener.py __name:=listener __log:=/home/ros/.ros/log/81a985dc-2506-11eb-987b-d17fa1d300a7/listener-6.log].
log file: /home/ros/.ros/log/81a985dc-2506-11eb-987b-d17fa1d300a7/listener-6*.lo

thanks