Adding virtual machines to a protection group fails with an error if you did not configure the appropriate inventory mappings.

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 configure the inventory mappings to map the devices of the virtual machine on the protected site to the corresponding devices on the recovery site.

Configure the inventory mappings as described in Site Recovery Manager Installation and Configuration.