Can you first tell me what happens when you bring up the context menu when RSS Feeds has focus and is selected? If you get the menu that includes Add a new RSS Feed as the third item (or at least that's the position in Outlook 2016) then we can go further.
If you don't have the RSS Feed address for the feed you want, you'll need to get that, too.
Brian - Windows 10, 64-Bit, Version 21H1, Build 19043
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