I downloaded turtlebot3 simulation package from http://emanual.robotis.com/docs/en/platform/turtlebot3/simulation/#turtlebot3-simulation-using-gazebo. Following the instruction installed the turtlebot3 simulation in my computer. My ros version is Melodic. At first, I can spawn turtlebot3emptyworld successfully. But after a day, the following error comes out:

Traceback (most recent call last):
  File "/opt/ros/melodic/lib/gazebo_ros/spawn_model", line 26, in <module>
    from gazebo_ros import gazebo_interface
  File "/opt/ros/melodic/lib/python2.7/dist-packages/gazebo_ros/gazebo_interface.py", line 9, in <module>
    from gazebo_msgs.msg import *
ImportError: No module named msg

I saw there are two posts about this, but unsolved, need urgent help. thanks