Unable to construct robot model when using Moveit with Open Manipulator

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I was trying to operate the robot arm Open Manipulator X with moveit's python interface. Similiar to this blog post. (It is written in Japanese) https://opensource-robotics.tokyo.jp/...

I created getinfo.py as below.

#!/usr/bin/env python

import rospy
import moveit_commander

node_name = "example"
rospy.init_node(node_name, anonymous=True)

group = moveit_commander.MoveGroupCommander("arm")

group.set_planning_time(600.0)

# Getting Initial Pose & RPY
pose_init = group.get_current_pose()
rospy.loginfo("Get Initial Pose\n{}".format(pose_init))

I run roscore, roslaunch open_manipulator_controller open_manipulator_controller.launch and then run this file with rosrun mypkg getinfo.py

However I got error like this

[ERROR] [1673607230.465912834]: Robot model parameter not found! Did you remap 'robot_description'?
[FATAL] [1673607230.466725336]: Unable to construct robot model. Please make sure all needed information is on the parameter server.
Traceback (most recent call last):
  File "src/getinfo.py", line 9, in <module>
    group = moveit_commander.MoveGroupCommander("arm")
  File "/opt/ros/melodic/lib/python2.7/dist-packages/moveit_commander/move_group.py", line 66, in __init__
    name, robot_description, ns, wait_for_servers
RuntimeError: Unable to construct robot model. Please make sure all needed information is on the parameter server.

My environment is

• Ubuntu 18.0.4
• Ros Melodic

Am I missing steps? Any helps would be appreciated.