robot_state_publisher and joint_state_publisher in launch file

edit

Assuming you're using xacro, one can do something like

from amenx_index_python import get_package_share_directory
from launch import LaunchDescription
from launch.substitutions import Command
from launch_ros.actions import Node
import os

def generate_launch_description():
    my_xacro = os.path.join(
        get_package_share_directory("PKG_NAME"),
        "urdf",
        "ROBOT_NAME.urdf.xacro",
    )

    return LaunchDescription(
        [
            Node(
                package="joint_state_publisher",
                executable="joint_state_publisher",
                name="joint_state_publisher",
            ),
            Node(
                package="robot_state_publisher",
                executable="robot_state_publisher",
                name="robot_state_publisher",
                parameters=[
                    {
                        "robot_description": Command(["xacro", " ", my_xacro]),
                    }
                ],
            ),
        ]
    )

Of course it is hard to know what your problem is without knowing more regarding the error output.

Note: this code was taken from a foxy pkg running on ubuntu 20.04.