Hi again.
There is not restriction on selection of syslog facility:
set system flow-accounting syslog-facility brownie
works but there is nota valid syslog options.
Adding a constraint resolves this:
<constraint> <regex>auth|authpriv|cron|daemon|kern|lpr|mail|mark|news|protocols|security|syslog|user|uucp|local0|local1|local2|local3|local4|local5|local6|local7|all</regex> </constraint>