Command[1] in section 3.2.9 is missing "-stack" option which should be used if default overcloud name is not being used.

Current command:

+++
openstack overcloud external-update run --tags online_upgrade
+++

Should be:
+++
openstack overcloud external-update run --stack <stack_name> --tags online_upgrade 
+++

[1]https://docs.redhat.com/en/documentation/red_hat_openstack_platform/16.2/html/keeping_red_hat_openstack_platform_updated/assembly_updating-the-overcloud_keeping-updated#proc_performing-online-database-updates_updating-overcloud