this is what the RHP looks for attachments -
make sure you actually have a (so called 'parent') item selected and not one of the files attached to it.
That should do it.
Whenever you scrape something from the web using Zotero (either through the item in the URL bar or through the "create new item from this page" automatically creates such a parent item)

Thank you for your comments and references.

My centre column item is indeed a Word document which is a transcription of an original handwritten narrative. I was trying to add a comment to the effect that a particular story in the narrative was confirmed by an entry in (website reference).

I now understand that I can achieve this by using either a Tag or a Related Item. Which should I choose and why?

As the link that I sent says:

Only regular items—with reference types such as books, journal articles, and newspaper articles—will show tabs in the right-hand column.

In order to add a comment (I assume you mean a note), your document needs to be an attachment to a regular item, not just a stand alone "attachment"( which, to my way of thinking, is somewhat of oxymoron, since it is not attached to anything).

So, if your document is a stand alone attachment, you need to create a regular item.

1. Click the green icon with a plus in it, and select the item type (probably "Document" or "Manuscript").

2. Then drag your attachment and drop it onto that item. Now, if you click on the main item, you will see some tabs in the right column.

3. Click on notes, Add, and create your comment, e.g. "This story in the narrative was confirmed by an entry in (website reference)".

4. Then I would navigate to that website, click on Create New Item from Current page.

5. You now have a new Web Page item.

6. Now, you can relate the comment from the previous item, to this web page.

No problem! Glad to help.