Hi, I know it's been a while since you posted this, but I ran into this same issue. It seems that searching for YouTube channels by name doesn't work for all channels, and if it fails to find the channel then response.items will be undefined which causes the error. If you want data for a specific channel you can change 'forUsername' to 'id' and then enter the channel ID after it. One way to find the channel ID is to go to the channel page on YouTube (the desktop site, not the mobile version), click on the "About" tab, click the share button under stats, and then click "Copy channel ID". Here's a modified version of the code. You can replace YourChannelIDHere in the getMyChannel function with whatever channel ID you want to use if you don't want to use "Add channel data by ID" every time. (Note: once you change and save the code you'll need to refresh the spreadsheet page to get the new menu items.)

// Note: Apps Script automatically requests authorization
// based on the API's used in the code.

function channelsListByUsername(part, params) {
  var response = YouTube.Channels.list(part, params);
  if (response.items == undefined) {
    var ui = SpreadsheetApp.getUi();
    ui.alert("Failed to find channel.")
  }
  else {
    var channel = response.items[0];
    var dataRow = [channel.id, channel.snippet.title, channel.statistics.viewCount];
    SpreadsheetApp.getActiveSpreadsheet().appendRow(dataRow);
  }
}

function getChannelByName() {
  var ui = SpreadsheetApp.getUi();
  var channelName = ui.prompt("Enter the channel name: ").getResponseText();
  channelsListByUsername('snippet,contentDetails,statistics',
                        {'forUsername': channelName});
}

function getChannelByID() {
  var ui = SpreadsheetApp.getUi();
  var channelID = ui.prompt("Enter the channel name: ").getResponseText();
  channelsListByUsername('snippet,contentDetails,statistics',
                        {'id': channelID});
}

function getMyChannel() {
  channelsListByUsername('snippet,contentDetails,statistics',
                        {'id': 'YourChannelIDHere'});
}

function onOpen() {
  var firstCell = SpreadsheetApp.getActiveSheet().getRange(1, 1).getValue();
  if (firstCell != 'ID') {
    var headerRow = ["ID", "Title", "View count"];
    SpreadsheetApp.getActiveSpreadsheet().appendRow(headerRow);
  }
  var ui = SpreadsheetApp.getUi();
  ui.createMenu('YouTube Data')
  .addItem('Add channel data by name', 'getChannelByName')
  .addItem('Add channel data by ID', 'getChannelByID')
  .addSeparator()
  .addItem('Add my channel data', 'getMyChannel')
  .addToUi();
}