NSX-T Data Center upgrade might fail if there is insufficient space in the bootbank or in the alt-bootbank on an ESXi host.
Unused VIBs on the ESXi host might be relatively large in size and therefore use up significant disk space. The unused VIBs can result in insufficient space in the bootbank or in the alt-bootbank during upgrade.
Uninstall the VIBs that are no longer required and free up additional disk space.
For more information on locating and deleting VIBs, see the VMware knowledge base article at https://kb.vmware.com/s/article/74864