can not run ROS after update from Ubuntu 18.04 to 20.04

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Hello Guys, I have updated my Ubuntu from 18.04 to 20.04, and reinstall ROS(melodic to noetic). When I finish the Install, I can not run the ROS anymore. when I run roscore, the following error occurs:

Traceback (most recent call last):  
File "/opt/ros/noetic/bin/roscore", line 37, in <module>
    from roslaunch.nodeprocess import DEFAULT_TIMEOUT_SIGINT,
DEFAULT_TIMEOUT_SIGTERM
File "/opt/ros/noetic/lib/python3/dist-packages/roslaunch/__init__.py", line 51, in <module>
    from . import core as roslaunch_core
File "/opt/ros/noetic/lib/python3/dist-packages/roslaunch/core.py", line 232, in <module>
    get_ros_root         = rospkg.get_ros_root
AttributeError: module 'rospkg' has no attribute 'get_ros_root'

Any ideas? Thank!!!