Document.querySelector returning a null value

Question

I am trying to get the patientNumber (ClinicA100-PF-TR1-P1) using querySelector. I keep getting a NULL value.

Here is a screenshot of the selectors (https://recordit.co/IypXuuXib0)

add_action( 'wp_head', function () { ?>
</script><script type="text/javascript">
  function getPatientNumber(){  
  var patientNumber = document.querySelector(".patientNumber");

  console.log(patientNumber);
  return patientNumber;
}

var patientNumber = getPatientNumber();
console.log(patientNumber);
_kmq.push(['identify', patientNumber]);
</script>
<?php } );

Thank you for any help you can provide.