One more question guys. I run python circuit2_turtlebot_lidar_qlearn.py again and I found that there is an error in roscore AttributeError: 'GazeboCircuit2TurtlebotLidarEnv' object has no attribute 'monitor'.
Then I followed this
and the error about can't launch the env still remain
Is there something wrong with this env? Is it related to the frozen of turtlebot?