NSX-T manager upgrade fails and the NSX-T manager cluster goes to degraded state.
You must perform the following steps to recover from NSX-T manager upgrade failure:
1. Login to the NSX-T manager UI and navigate to System > Lifecycle Management > Upgrade and Click Continue to Upgrade.
OR,
If the above holds true then reboot both the node VMs on which the MONITORING service is DOWN by running the command reboot on the NSX-T appliance's CLI.
3. After the VMs are rebooted, verify that the MONITORING service is UP on the nodes that were rebooted and
NSX-T cluster status is Stable under System > Configuration > Appliances.
4. Retry the failed upgrade from SDDC manager (Updates/Patches tab) and SDDC manager will report that the NSX-T upgrade is successful.
5. You can now run precheck and continue to upgrade vCenter server and ESXi.