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Custom child table script not functioning

Hi, I want to change the column heading of a child table by using client script but somehow there are no changes affected to the child table after I created the script. The first picture is the current(default state of my child table).

The second picture is how I want my child table to become. I just want to add another row of column heading.

The third picture is the scripts that I wrote but this code doesn’t make any changes to my child table. Can anyone help me to spot where is my mistake is or anyone knows another way for me to achieve the desired child table?