Echo signature and php function

How To
1

I am trying to get a custom function to work on my dev site. I created a test function, first in WPCodebox(that didn’t work) then directly into functions.php The function is in the file. however, the settings says it doesn’t exist. Any idea how I can fix and run this function as a test?

add_filter( 'bricks/code/echo_function_names', function() {
  return [
    'date',
    'test_function', // function does not exist
  ];
} );