Convert Google Doc to PDF using Google Script Editior

It is failing because the folder is not defined. If you replace it with DriveApp, the PDF will be created in the root folder and your function would work. You can also show the full URL in the message box.

function convertPDF() {
  doc = DocumentApp.getActiveDocument();
  var ui = DocumentApp.getUi();
  var result = ui.alert(
      'Save As PDF?',
      'Save current document (Name:'+doc.getName()+'.pdf) as PDF',
      ui.ButtonSet.YES_NO);
  if (result == ui.Button.YES) {
    docblob = DocumentApp.getActiveDocument().getAs('application/pdf');
    /* Add the PDF extension */
    docblob.setName(doc.getName() + ".pdf");
    var file = DriveApp.createFile(docblob);
    ui.alert('Your PDF file is available at ' + file.getUrl());
  } else {
    ui.alert('Request has been cancelled.');
  }
}

To save the PDF in the original directory:

function convertPDF() {
  doc = DocumentApp.getActiveDocument();
  // ADDED
  var docId = doc.getId();
  var docFolder = DriveApp.getFileById(docId).getParents().next().getId();
  // ADDED
  var ui = DocumentApp.getUi();
  var result = ui.alert(
      'Save As PDF?',
      'Save current document (Name:'+doc.getName()+'.pdf) as PDF',
      ui.ButtonSet.YES_NO);
  if (result == ui.Button.YES) {
    docblob = DocumentApp.getActiveDocument().getAs('application/pdf');
    /* Add the PDF extension */
    docblob.setName(doc.getName() + ".pdf");
    var file = DriveApp.createFile(docblob);
    // ADDED
    var fileId = file.getId();
    moveFileId(fileId, docFolder);
    // ADDED
    ui.alert('Your PDF file is available at ' + file.getUrl());
  } else {
    ui.alert('Request has been cancelled.');
  }
}

And add this generic function

function moveFileId(fileId, toFolderId) {
   var file = DriveApp.getFileById(fileId);
   var source_folder = DriveApp.getFileById(fileId).getParents().next();
   var folder = DriveApp.getFolderById(toFolderId)
   folder.addFile(file);
   source_folder.removeFile(file);
}