This error message usually appears when you are trying to activate macOS (or an Apple server application like macOS Server) and the machine cannot establish a valid, secure connection with Apple's activation servers.
After the reboot, reconnect to Wi-Fi. Wait about ; the Mac may then activate successfully on its own. 4. Advanced Troubleshooting
: Ensure the "macOS Device Activation" service is operational by checking the Apple System Status page from another device. Restart and Retry