When a Compute Engine instance cannot boot due to a configuration error, standard methods like SSH or SCP may not work because the SSH service depends on the system reaching a normal run level. Similarly, remote desktop clients require GUI services which are unlikely to be running. The correct approach is to use the serial port console, which provides low-level access to the instance even if it fails to boot properly. This allows direct troubleshooting and editing of configuration files without relying on the instance's network services.