I have the same issue also, it seems that with yoga/stable channel of nova-compute charm the nova.conf is rendered using /var/lib/juju/agents/unit-nova-compute-6/charm/templates/train/nova.conf template which is old and has no [vnc] section.
I have the same issue also, it seems that with yoga/stable channel of nova-compute charm the nova.conf is rendered using /var/lib/ juju/agents/ unit-nova- compute- 6/charm/ templates/ train/nova. conf template which is old and has no [vnc] section.