"roslaunch turtle_tf2 turtle_tf2_demo.launch" Error

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tf2 has been installed, but the command line of the lawsuit is wrong.Only one turtle is shown. The error message is as follows:

File "/opt/ros/noetic/lib/turtle_tf2/turtle_tf2_broadcaster.py", line 35, in <module> import rospy File "/opt/ros/noetic/lib/python3/dist-packages/rospy/__init__.py", line 47, in <module> from std_msgs.msg import Header File "/opt/ros/noetic/lib/python3/dist-packages/std_msgs/msg/__init__.py", line 1, in <module> from ._Bool import * File "/opt/ros/noetic/lib/python3/dist-packages/std_msgs/msg/_Bool.py", line 6, in <module> import genpy File "/opt/ros/noetic/lib/python3/dist-packages/genpy/__init__.py", line 34, in <module> from . message import Message, SerializationError, DeserializationError, MessageException, struct_I File "/opt/ros/noetic/lib/python3/dist-packages/genpy/message.py", line 48, in <module> import yaml ImportError: No module named yaml Traceback (most recent call last): File "/opt/ros/noetic/lib/turtle_tf2/turtle_tf2_broadcaster.py", line 35, in <module> import rospy File "/opt/ros/noetic/lib/python3/dist-packages/rospy/__init__.py", line 47, in <module> from std_msgs.msg import Header File "/opt/ros/noetic/lib/python3/dist-packages/std_msgs/msg/__init__.py", line 1, in <module> from ._Bool import * File "/opt/ros/noetic/lib/python3/dist-packages/std_msgs/msg/_Bool.py", line 6, in <module> import genpy File "/opt/ros/noetic/lib/python3/dist-packages/genpy/__init__.py", line 34, in <module> from . message import Message, SerializationError, DeserializationError, MessageException, struct_I File "/opt/ros/noetic/lib/python3/dist-packages/genpy/message.py", line 48, in <module> import yaml ImportError: No module named yaml Traceback (most recent call last): File "/opt/ros/noetic/lib/turtle_tf2/turtle_tf2_listener.py", line 34, in <module> import rospy File "/opt/ros/noetic/lib/python3/dist-packages/rospy/__init__.py", line 47, in <module> from std_msgs.msg import Header File "/opt/ros/noetic/lib/python3/dist-packages/std_msgs/msg/__init__.py", line 1, in <module> from ._Bool import * File "/opt/ros/noetic/lib/python3/dist-packages/std_msgs/msg/_Bool.py", line 6, in <module> import genpy File "/opt/ros/noetic/lib/python3/dist-packages/genpy/__init__.py", line 34, in <module> from . message import Message, SerializationError, DeserializationError, MessageException, struct_I File "/opt/ros/noetic/lib/python3/dist-packages/genpy/message.py", line 48, in <module> import yaml ImportError: No module named yaml [turtle1_tf2_broadcaster-3] process has died [pid 21227, exit code 1, cmd /opt/ros/noetic/lib/turtle_tf2/turtle_tf2_broadcaster.py __name:=turtle1_tf2_broadcaster __log:=/home/liudian/.ros/log/1f4727f4-3c45-11eb-8f80-a7fd921df6fa/turtle1_tf2_broadcaster-3.log]. log file: /home/liudian/.ros/log/1f4727f4-3c45-11eb-8f80-a7fd921df6fa/turtle1_tf2_broadcaster-3.log [turtle2_tf2_broadcaster-4] process has died [pid 21228, exit code 1, cmd /opt/ros/noetic/lib/turtle_tf2/turtle_tf2_broadcaster.py __name:=turtle2_tf2_broadcaster __log:=/home/liudian/.ros/log/1f4727f4-3c45-11eb-8f80-a7fd921df6fa/turtle2_tf2_broadcaster-4.log]. log file: /home/liudian/.ros/log/1f4727f4-3c45-11eb-8f80-a7fd921df6fa/turtle2_tf2_broadcaster-4.log [turtle_pointer-5] process has died [pid 21234, exit code 1, cmd /opt/ros/noetic/lib/turtle_tf2/turtle_tf2_listener.py __name:=turtle_pointer __log:=/home/liudian/.ros/log/1f4727f4-3c45-11eb-8f80-a7fd921df6fa/turtle_pointer-5.log]. log file: /home/liudian/.ros/log/1f4727f4-3c45-11eb-8f80-a7fd921df6fa/turtle_pointer-5*.log