Googling the error message took me to this thread:
https://community.ipfire.org/t/ipfire-167-no-ruleset/7839 which had a slightly different problem, but the cause seems to have been the same.
It appears the wrong owner/group have been set for the file /var/ipfire/suricata/providers-settings. On my system they were root:root when they should have been nobody:nobody
Running the following command from the SSH console logged in to my firewall fixed the problem for me:
chown nobody:nobody /var/ipfire/suricata/providers-settings