After a major upgrade to the operating system, ARD never seems to start on the client machine. This usually requires having someone physically at the machine to toggle Screen Sharing or Remote Management off and on again. To get around this, you’ll want to be sure Remote login is enabled so after the upgrade you can SSH into the machine. This works to re-enable screen sharing for applications like Screenconnect as well.
After upgrading the machine, discover its IP address on the network, SSH into the machine as an administrator and type the following.
sudo pkill -u -2 ARDAgent