The solution to her particular problem was to kill the AirPlay process running on her Mac.

From what I can tell, after she did that AirPlay doesn’t automatically shut itself off, and it was preventing the screensaver from starting. The short story of the problem (and solution) is that she had used Apple’s AirPlay to mirror her computer screen to her AppleTV. The same problem happened to a friend, and after a LOT of research, we were able to work through it to get her Mac screensaver working again. Now when I'm finished working, I can click the icon in the Dock and quit the window in ARD, and I leave the machine with an active screensaver.If you have a problem where your Mac/ macOS screensaver won’t start, I hope to be able to help you in this article. I then saved the workflow as an application (File: Save As, and set the File Format pop-up to Application), and put it into Dock on the remote computer. Once there, I chose ScreenSaverEngine.app as the application to launch. Using the to pop-up menu in the Launch Application action, I chose Other (all the way at the bottom), and then navigated to /System -> Library -> Frameworks -> amework -> Versions -> A -> Resources.

Next, I chose Finder from the Library column, and dragged the Launch Application action to the work area. I set the pause time to five seconds, to give myself a bit of time to quit ARD. Using Automator, I chose Automator from the Library column, and dragged Pause from the Action column into the work area. When I use Apple Remote Desktop (ARD) to reach another Mac, I like to leave the computer with an active screen saver after I access it.

Start screen saver mac