Basic Kinematics of Mobile Robots rosject can not launch teleop

for part I, when launch the teleop, the error is:

user:~$ roslaunch turtlebot3_teleop turtlebot3_teleop_key.launch... logging to /home/user/.ros/log/277a216a-ae5e-11ec-8978-0242c0a81007/roslaunch-2_xterm-9927.logChecking log directory for disk usage. This may take a while.Press Ctrl-C to interrupt
Done checking log file disk usage. Usage is <1GB.
RLException: Invalid <arg> tag: environment variable 'TURTLEBOT3_MODEL' is not
Arg xml is <arg name="model" default="$(env TURTLEBOT3_MODEL)" doc="model type
[burger, waffle, waffle_pi]"/>The traceback for the exception was written to the log file

how to solve it! thx!

You need to set the environment variable: export TURTLEBOT3_MODEL=burger before launching the teleop