If you can’t share Mac computer screens
If you’re having problems sharing your screen with another Mac or viewing a shared screen on your Mac, there are several things to check.
Make sure you turned on screen sharing or remote management and set up sharing permissions on the Mac whose screen you want to share. See Turn Mac screen sharing on or off.
If you’re trying to share the screen of another Mac, on that Mac choose Apple menu
in the sidebar, click Sharing on the right, click the Info button 
next to Screen Sharing, then make sure you’re in the list of users who have permission to share its screen.Make sure neither Mac is in sleep. If necessary, adjust the sleep settings for each Mac.
Make sure both computers are connected to the same network.
To check your connection, choose Apple menu > System Settings, then click Network
in the sidebar (you may need to scroll down). On the right, the indicator next to your type of network connection should be green. For a Wi-Fi connection, click Wi-Fi to check the network name.
