This is a result of the virtual machine's hardware version being under version 7, where Change Block Tracking (CBT) was introduced. If your VMware VM is still running hardware version 4 then you will not be able to bring the VM underneath agent-less protection in Rapid Recovery. Rapid Recovery requires the use of CBT for the agent-less protection and since CBT was first introduced in VMware's hardware version 7, your virtual machines must be running version 7 or higher in order to use Rapid Recovery's agent-less protection.
VMware's Change Block Tracking documentation can be found here:
The resolution for this is to upgrade the hardware version of your virtual machine(s). This can be done a number of methods, here are two ways to do so.
NOTE: This is a permanent change to your VM, so it is recommended that you take a VMware snapshot of your VM first, then upgrade the virtual hardware version to validate that there aren't any adverse affects. If you do you can then power off your VM and revert the snapshot to return you to a pre-hardware update VM.
To do this from within the vSphere client: (through vSphere you can only upgrade to the 'newest hardware version on the host, you can choose a specific version)
To do this from within the web client: (Through the web client you can specify any hardware version that the host supports)