Mac Mini connection failed - screen sharing is not permitted

I had an old mac mini running OS 10.13 I used as a music server. To have my Macbook share its screen, I'd click the Mini user name in the finder sidebar, click share screen, enter the Mini's user name and password, and sharing would begin.

I've now replaced the old one with a new mini (OS 15). On the mini, screen sharing is on, allow access for all users. Now instead of seeing the Mini's user name in the finder sidebar, I see Network, click on that and I see the mini, click Share. Now if I enter the Mini's user name and password, it says Connection failed - screen sharing is not permitted . . . So I'm doing the same thing I did with the old mini, but it doesn't work with the new one running OS 15. How am I supposed to set this up?

[Re-Titled by Moderator]