After activating the Backup and DR service, it takes a little time for the backup appliance to get registered with the management server. If you login right after the management server comes online, you may see this error when trying to perform a task that needs a backup appliance.
For SQL Server, are you looking to restore a SQL Server database from an on-premises datacenter into GCP? If so, is this going into a self-managed SQL Server instance running in Compute Engine, or into a SQL Server instance running in Cloud SQL? In both scenarios, you would not use the Backup and DR service. If you are using a self-managed SQL Server instance in a Windows VM in Compute Engine, then you could use Backup and DR to enable backups and recovery for that DB after you have created it in your instance. Here is a video that walks you through the setup for backups of a self-managed SQL Server instance. [Video]