Have your account received any messages? That may be a cause. If IMAP, rightclick on account name in folder pane and click 'subscribe' and see if you can set them up there.

Many thanks David for your quick response to my request for help! BUT, when I do as you recommend and tick the boxes nothing happens. Curious - so do you have any further thoughts?

I am unsure on whether the folders appear until there is a message there. And on that, I am tapped out. I wish you well and there are others here with more technical expertise who may respond.

Thanks David

You're very welcome. I wish I could have been more helpful.

For what it's worth, a folder with content still does not appear!

Go to View > Folders and verify if ✔︎ All Folders is selected.

Thanks Mapenzi - but where do I find the View option?

For Windows users the menu bar is hidden by default. Holding the Alt key will display it ... https://support.mozilla.org/en-US/kb/display-thunderbird-menus-and-toolbar#thunderbird:win11:tb147

Well, that gives me Unified Folders but not sub-folders for my new account!