The same issue for me. I configured master/backup firewalls, CARP is working properly, but config sync is not working, because - "The backup firewall is not accessible or not configured."
All was done by this instruction - https://docs.opnsense.org/manual/how-tos/carp.html
All was done by this instruction - https://docs.opnsense.org/manual/how-tos/carp.html