Apps Script: TypeError: Cannot read properties of undefined (reading 'range')

This is a plain JavaScript error from the V8 runtime: the code tried to read a property of a variable that is undefined. In Apps Script it appears most often in trigger functions such as onEdit(e), onFormSubmit(e) or doGet(e), and the property in parentheses (range, source, values, parameter) is a property of the event object e.

The classic cause is clicking Run on the trigger function in the editor. When a trigger fires, Apps Script passes an event object; when you run the function by hand, nothing is passed, so e is undefined and e.range fails on the first line.

The function is usually fine. It just was not started by the event it was written for.

function onEdit(e) {
  if (!e || !e.range) return;   // started from the editor
  const sheet = e.range.getSheet();
  ...
}

function testOnEdit() {
  const range = SpreadsheetApp.getActiveSheet().getRange('B2');
  onEdit({ range, value: range.getValue(), source: SpreadsheetApp.getActive() });
}

Keep event handlers thin: validate the event object, then call a normal function that does the work and can be tested independently. Name installable-trigger handlers clearly so it is obvious which event they expect.