tf tutorial start_demo.launch wtf

edit

Are you sure you copy and pasted it correctly? The problem is that you're missing some of the node tags. Turn this

<launch>
    <!-- Turtlesim Node-->

    <node pkg="turtlesim" type="turtlesim_node" name="sim"/>
    <node pkg="turtlesim" type="turtle_teleop_key" name="teleop" output="screen"/>

    <!-- Missing opening <node> -->
        <param name="turtle" type="string" value="turtle1" />
    </node>

    <!-- Missing opening <node> -->
        <param name="turtle" type="string" value="turtle2" /> 
    </node>

</launch>

into this

<launch>
  <!-- Turtlesim Node-->
  <node pkg="turtlesim" type="turtlesim_node" name="sim"/>
  <node pkg="turtlesim" type="turtle_teleop_key" name="teleop" output="screen"/>

  <node name="turtle1_tf_broadcaster" pkg="learning_tf" type="turtle_tf_broadcaster.py" respawn="false" output="screen" >
    <param name="turtle" type="string" value="turtle1" />
  </node>
  <node name="turtle2_tf_broadcaster" pkg="learning_tf" type="turtle_tf_broadcaster.py" respawn="false" output="screen" >
    <param name="turtle" type="string" value="turtle2" /> 
  </node>

</launch>

Notice how you're missing two

<node name="turtle1_tf_broadcaster" pkg="learning_tf" type="turtle_tf_broadcaster.py" respawn="false" output="screen" >

Edit:

To create a proper XML you need to have:

• both opening and closing tags (e.g. <node> and </node>, respectively), or
• self-closing tags (e.g., <node/> note the / before the >)

If you don't properly open and close tags then your file is not properly formatted and you'll get an error.