If update finished with status FAILED then the guide should specify which logs to check, here or in Troubleshooting section https://access.redhat.com/documentation/en-us/red_hat_openstack_platform/13/html/fast_forward_upgrades/assembly-preparing_for_openstack-platform_upgrade#annotations:6c42d513-547b-4133-ba7a-fa19b4067a11
Nadav and I discussed this BZ and agreed this would be an ideal procedure for a Troubleshooting section.