The error message that a Pages template couldn't be opened and "The required index file is missing" usually occurs when you are using an older version of Pages. Sorry, but we don't support Pages prior to version 5 (2013).
Instead, you can try to convert an Avery template for Microsoft Word to an Apple Pages document:
- After you download the Word Template from avery.com, the template will be in your Downloads Folder.
- Find the Word template from the Finder on your Mac.
- Right-click the file, then click Open with and choose Pages.