Thanks you. The FATAL message is not clear. Yes, It is helpful if someone with more knowledge of the connection code.
[postgres@pgserver ~]$ /opt/PostgreSQL/9.5/bin/pg_basebackup --format=t --pgdata=online_backups1 -p 5432 -U backup_admin -x -z --verbose
pg_basebackup: could not connect to server: FATAL: must be superuser or replication role to start walsender