Google Sheets script that produces the time, date, and user of the last time the document was changed

When I run the script I get the error "TypeError: Cannot find function getLastUpdated in object Sheet.
getLastModified @ Code.gs:3"

function getLastModified() {
  var sheet = SpreadsheetApp.getActiveSpreadsheet().getActiveSheet();
  var lastModified = sheet.getLastUpdated();
  var user = Session.getActiveUser().getEmail();
  var timeZone = Session.getTimeZone();
  var formattedDate = Utilities.formatDate(lastModified, timeZone, "MM/dd/yyyy HH:mm:ss");
  var message = "Last modified by " + user + " on " + formattedDate;
  sheet.getRange("I1").setValue(message);
}

I am trying to run a script to automaitcally return a message in cell I1 when the sheet is updated.

The error your'e getting is because getLastUpdated() is not a valid method for a sheet class. You should use the onEdit function as it's an event and just grab your data.

Example:

function onEdit(e) {
  var sheet = SpreadsheetApp.getActiveSpreadsheet().getActiveSheet();
  var rightNow = new Date();
  var user = Session.getActiveUser().getEmail();
  var timeZone = Session.getTimeZone();
  var formattedDate = Utilities.formatDate(rightNow, timeZone, "MM/dd/yyyy HH:mm:ss");
  var message = "Last modified by " + user + " on " + formattedDate;
  sheet.getRange("I1").setValue(message);
}