I'm using New Outlook (version 16.47) on an iMac using Catalina and I'm on the Beta programme.
I run about five different email accounts on Outlook using a variety of iCloud, Outlook.com, IMAP & Gmail accounts. All of them have this problem. When I select an email it appears in the preview pane. However, when I use any button to reply to that email I get a message in the Preview saying 'No Conversation Selected. Select a conversation to read'. I've tried turning the conversation view on and off (its normally off). If conversation view is on this doesn't happen, but I want it off.