Master-slave replication doesn't work on PostgreSQL 9.6
You told it look for the data in '/srv/postgresql'.  But then you told pg_basebackup to clone the data into '/var/lib/postgresql/9.6/main', not '/srv/postgresql'.

When you started postgresql up again, you (apparently) started up the database instance whose data is still in '/srv/postgresql', which is not the clone you had just made.

Also, turning "archive_mode" on but then providing it with a dummy "archive_command" is not helpful and can only cause confusion and mis-reported errors.  And "pg_basebackup" has a -R option for writing out the recovery.conf itself, it would probably be best to use that.