When run with this command, pgbadger finds no queries, even though there are slow queries logged in the database log.
pgbadger --dbname foo /path/to/logfile --format stderr --prefix '%m [%p] '
Example log entry
2020-04-08 07:58:42.014 BST [1505890] LOG: duration: 119.079 ms statement: SELECT "bar".id FROM "bar" WHERE (("bar"."baz" = 0.0) AND ("bar"."quux" = 0.0)) ORDER BY "bar"."id"
The problem was passing the --dbname flag and its argument. The log prefix '%m [%p] ' does not include the database name, so pgbadger, presumably, is unable to find any statements logged against the provided database name and reports accordingly.
The solution is to either not pass --dbname or modify the log prefix in postgresql.conf to include the database name (for example '%m [%p] %d '), reload the server config and wait for new entries in the log.
I found this on an Openstack Fedora vm, where '%m [%p] ' was the default log prefix.