“No rebuild capacity available” warning appears to Nutanix clusters when amount of free disk space in clusters is lower than capacity of a single node. This means when you loose one node it is not possible to fully rebuild data resiliency. There are options to fix this – free up space or add more nodes to your cluster. You can free up space by removing obsolete VMs or reclaim dead space within running VMs. I covered here how to reclaim space within VMs.
To check how much storage capacity single node provides go to Hardware -> Table -> select one host -> check “Storage Capacity” value.