ros2 launch turtlebot3_gazebo empty_world.launch.py error

[jsb9945]

Hello!
My environment is ros2 foxy ubuntu 20.04.
I executed this command.

ros2 launch turtlebot3_gazebo empty_world.launch.py

then, i got this error

[INFO] [launch]: All log files can be found below /home/csrl/.ros/log/2023-02-01-12-30-26-169214-csrl-System-Product-Name-61039
[INFO] [launch]: Default logging verbosity is set to INFO
Task exception was never retrieved
future: <Task finished name='Task-2' coro=<LaunchService._process_one_event() done, defined at /opt/ros/foxy/lib/python3.8/site-packages/launch/launch_service.py:226> exception=InvalidLaunchFileError('py')>
Traceback (most recent call last):
File "/opt/ros/foxy/lib/python3.8/site-packages/launch/launch_description_sources/any_launch_file_utilities.py", line 53, in get_launch_description_from_any_launch_file
return loader(launch_file_path)
File "/opt/ros/foxy/lib/python3.8/site-packages/launch/launch_description_sources/python_launch_file_utilities.py", line 62, in get_launch_description_from_python_launch_file
launch_file_module = load_python_launch_file_as_module(python_launch_file_path)
File "/opt/ros/foxy/lib/python3.8/site-packages/launch/launch_description_sources/python_launch_file_utilities.py", line 37, in load_python_launch_file_as_module
loader.exec_module(mod)
File "", line 848, in exec_module
File "", line 219, in _call_with_frames_removed
File "/home/csrl/aws_ws/install/turtlebot3_gazebo/share/turtlebot3_gazebo/launch/empty_world.launch.py", line 28, in
TURTLEBOT3_MODEL = os.environ['TURTLEBOT3_MODEL']
File "/usr/lib/python3.8/os.py", line 675, in getitem
raise KeyError(key) from None
KeyError: 'TURTLEBOT3_MODEL'

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
File "/opt/ros/foxy/lib/python3.8/site-packages/launch/launch_service.py", line 228, in _process_one_event
await self.__process_event(next_event)
File "/opt/ros/foxy/lib/python3.8/site-packages/launch/launch_service.py", line 248, in __process_event
visit_all_entities_and_collect_futures(entity, self.__context))
File "/opt/ros/foxy/lib/python3.8/site-packages/launch/utilities/visit_all_entities_and_collect_futures_impl.py", line 45, in visit_all_entities_and_collect_futures
futures_to_return += visit_all_entities_and_collect_futures(sub_entity, context)
File "/opt/ros/foxy/lib/python3.8/site-packages/launch/utilities/visit_all_entities_and_collect_futures_impl.py", line 45, in visit_all_entities_and_collect_futures
futures_to_return += visit_all_entities_and_collect_futures(sub_entity, context)
File "/opt/ros/foxy/lib/python3.8/site-packages/launch/utilities/visit_all_entities_and_collect_futures_impl.py", line 38, in visit_all_entities_and_collect_futures
sub_entities = entity.visit(context)
File "/opt/ros/foxy/lib/python3.8/site-packages/launch/action.py", line 108, in visit
return self.execute(context)
File "/opt/ros/foxy/lib/python3.8/site-packages/launch/actions/include_launch_description.py", line 130, in execute
launch_description = self.__launch_description_source.get_launch_description(context)
File "/opt/ros/foxy/lib/python3.8/site-packages/launch/launch_description_source.py", line 84, in get_launch_description
self._get_launch_description(self.__expanded_location)
File "/opt/ros/foxy/lib/python3.8/site-packages/launch/launch_description_sources/any_launch_description_source.py", line 53, in _get_launch_description
return get_launch_description_from_any_launch_file(location)
File "/opt/ros/foxy/lib/python3.8/site-packages/launch/launch_description_sources/any_launch_file_utilities.py", line 56, in get_launch_description_from_any_launch_file
raise InvalidLaunchFileError(extension, likely_errors=exceptions)
launch.invalid_launch_file_error.InvalidLaunchFileError: Caught exception when trying to load file of format [py]: 'TURTLEBOT3_MODEL'

What's the problem? please help me

[saiaravind19]

The mode which needs to be spawned is not provided so is the error.

1. To spawn turtlebot use
   export TURTLEBOT3_MODEL=burger
ros2 launch turtlebot3_gazebo empty_world.launch.py

2.To spawn waffle use
export TURTLEBOT3_MODEL=waffle
ros2 launch turtlebot3_gazebo empty_world.launch.py

In case you always use turtlebot then use the below command
echo "export TURTLEBOT3_MODEL=burger" >> ~/.bashrc