There are two main reason why your virtual machine will not be listed:
The Host for the VM is not listed because it is not licensed
Navigate to Options > Licensing Settings and select manage hosts check the Host is licensed. If it is not, this will prevent the host from showing and the VM will not be listed.
The VM already has a backup job
It is only possible to protect a machine with one backup job. You can however still create a replication job to protect your machine with Hot Standby.