How to check why a PostgreSQL server won't start

journalctl -u postgresql-15.service

Example output:

Dec 27 07:15:50 psl0part-prod-pgsql02.skead.no systemd[1]: Unit postgresql-15.service entered failed state.
Dec 27 07:15:50 psl0part-prod-pgsql02.skead.no systemd[1]: postgresql-15.service failed.
Dec 27 07:22:01 psl0part-prod-pgsql02.skead.no systemd[1]: Starting PostgreSQL 15 database server...
Dec 27 07:22:01 psl0part-prod-pgsql02.skead.no postmaster[65528]: 
2024-12-27 06:22:01.703 GMT [65528] 
LOG:  syntax error in file "/path/to/file/postgresql.auto.conf" line
Dec 27 07:22:01 psl0part-prod-pgsql02.skead.no systemd[1]: postgresql-15.service: main process exited, code=exited, status=1/FAILURE
Dec 27 07:22:01 psl0part-prod-pgsql02.skead.no systemd[1]: Failed to start PostgreSQL 15 database server.
Dec 27 07:22:01 psl0part-prod-pgsql02.skead.no systemd[1]: Unit postgresql-15.service entered failed state.
Dec 27 07:22:01 psl0part-prod-pgsql02.skead.no systemd[1]: postgresql-15.service failed.

In this case, the error seems to be in an error in the file postgresql.auto.conf.