The most common operational problem.
THE FIRST COMMAND
systemctl status for the service. It reports the failure and usually several relevant log lines. Read them rather than guessing.
THE COMMON CAUSES
A configuration syntax error, usually after an edit Another process already using the port Insufficient memory to start A missing file the service expects Permissions preventing access to a required file A dependency service not running
DIAGNOSING
Run the service's configuration test command if it has one. Check what is listening on the expected port. Check free memory. Check the service's own error log, which is frequently more detailed than the systemctl output.
AFTER A CONFIGURATION CHANGE
Suspect the change. Compare against your backup copy of the file, which you took before editing.
AFTER A SYSTEM UPDATE
A configuration file may have been replaced, or an option deprecated. Check the service's log for warnings about unrecognised directives.
IF IT STARTS AND IMMEDIATELY STOPS
Read the log from the moment of the attempt. Something is failing during initialisation.
