The SQL Server instance might not be listed due to insufficient permissions during the installation of the BDRShield Client Files Application, preventing access to SQL server entries in the system registry.

1. Change Service Logon Account:

1. Change the `BDRShield_Client_Files_Application` service logon account to an Administrative account from the Local System Account.

2. Ensure Necessary Plugin Files Are Present:

1. This issue may occur if the necessary plugin files are missing from the installation location. It is  due to insufficient permissions for BDRShield_Client_Files_Application to access SQL Server entries in the system registry.
2. To resolve this,
   
1. Download the files `AdvMSSQLServer.sgpl` and `MSSQLServer.sgpl` 
   
2. Move these files to the following location:
   

      C:\Program Files\Vembu\VembuNetworkBackup\plugins

1.  Once the files are placed, restart the "BDRShield_Client_Files_Application" service.
   

3. List the SQL Server Instance:

1. Try to list the SQL server instance again.
2. If you still face issues, contact BDRShield Support
   

By changing the service logon account to an administrative account and ensuring the necessary files are in place, you can resolve the issue and list the SQL Server instance successfully.