Trying to run a script that moves the starting time of my markers

Hello I want to move the starting time of all my markers backwards half a second.
Chat GPT came up with this:

// Define the time offset (in seconds) to move the markers
var timeOffset = -0.5; // Half a second backward

// Get the active session
var activeSession = app.activeSession;

// Check if there's an active session
if (activeSession) {
  // Get all markers in the active session
  var markers = activeSession.markers;

  // Iterate through all markers
  for (var i = 0; i < markers.numMarkers; i++) {
    var marker = markers.getMarker(i);

    // Calculate the new position for the marker
    var newStartTime = marker.startTime + timeOffset;

    // Update the marker's position
    marker.startTime = newStartTime;
  }

  // Refresh the view to see the changes
  activeSession.view.refresh();
} else {
  alert("No active session found. Please open an audio file in Adobe Audition.");
}

my issue is that i keep getting the message  "No active seesionfound. please open an audio file in Adobe Audition." Any suggestions?