Here it is more than a year later and I'm still dealing with this. What's the story, Microsoft? Just unwilling to support people w/Yahoo accounts?
Yes, as John says, you need to switch to Outlook Mail for Windows. I was working on the old version when I got a message saying try the new Outlook version and there should be a toggle in the top right of your screen within Mail. See this link https://support.microsoft.com/en-us/office/getting-started-with-the-new-outlook-for-windows-656bb8d9-5a60-49b2-a98b-ba7822bc7627
It says somewhere that it doesn't work with Yahoo. It does! There are some videos on Youtube showing the new features but it handles similarly to the old Mail. Microsoft wont use Youtube of course to explain to the world what is going on. Seems bizarre they have no media outlet for us users to turn to and they seem to accept we should work it out for ourselves and then tell each other.