How to use tf.transformPose of tf mpdule?

edit

Hello, I'm using ROS melodic of Ubuntu 18.04.

I recently heard that tf module has the special class function which transforms a specific pose into the pose of another specific frame.

I tried calling tf.TransformerROS.transformPose, but I've just gotten only this error:

"/sensor_frame" passed to lookupTransform argument target_frame does not exist

. Of course, I double-checked that /sensor_frame was practically running. (I tried

$ rosrun tf tf_echo

and also checked it on rviz tool.)

Below is my code,

import rospy
import tf
from geometry_msgs.msg import PoseStamped


def talker():
  pub = rospy.Publisher('/that', PoseStamped, queue_size = 10)
  rate = rospy.Rate(10)

   # goaler = tf.TransformerROS()
   goaler = tf.TransformListener()

   while not rospy.is_shutdown():
      goal = PoseStamped()
      goal.pose.position.x = 5.0
      goal.pose.position.y = 5.0

      # ret = goaler.transformPose("lidar_sensor_link", goal)
      ret = goaler.transformPose("/sonar2_link", goal)

      pub.publish(ret)
      rate.sleep()



 if __name__ == '__main__':
   rospy.init_node('pracd')

    try:
        talker()
    except rospy.ROSInterruptException:
        pass

(Just so you know, I referred to here)

I'm not familiar with tf, so I 'm suffered from fixing..

What should I do to coordinate transformation works?

Thanks inadvance :)