Adding virtual machines to a protection group fails with an error if you did not map the devices of the virtual machine.

When you add a virtual machine to a protection group, you see the error Unable to protect VM 'virtual machine name' due to unresolved devices.

You did not map the devices of the virtual machine on the protected site to the corresponding devices on the recovery site.

Configure the protection settings of the virtual machine as described in Modifying the Settings of a Protected Virtual Machine.