The issue arises when VembuBDR or VembuVMBackup cannot access the target machine's admin$ share, which is a universal share path typically required for copying the VembuInstallationService installer onto the target machine.

Follow the steps below to troubleshoot and resolve the "The Remote Server is Not Reachable" issue:

1. UNC Validation:

1. Manually verify admin$ share access from the machine where "VembuBDR" or "VembuVMBackup" is installed.
2. Open the Run dialog and type: \\<IP>\admin$
3. Replace <IP> with the hostname or IP address of the target machine (e.g., \192.168.100.100\admin$).
4. If successful, proceed to the next step.

2. Troubleshoot Network Connectivity:

If there are network connectivity issues, follow these steps:

     Case 1: Firewall Settings

         1. Open Control Panel.

         2. Navigate to System and Security > Windows Firewall > Advanced Settings.

         3. In Windows Firewall with Advanced Security, click Inbound Rules.

         4. Click New Rule, choose Predefined, select "Remote Service Management," and enable it.

     Case 2: Ensure Services are Running

      Verify that the following services are running on the target machine:

1. Server Service
2. Windows Remote Management (WS-management)
   

Note: After verifying Case 1 and Case 2, try UNC access again to confirm successful UNC validation.

3. Confirmation Test using WBEM Utility:

1. Open "wbemtest" on your server/client machine.
2. In the namespace text box, provide the target hostname/IP: \\IP\root\cimv2 (e.g., \192.168.100.100\root\cimv2).
3. Provide the username and password and click Connect.
4. If there is an error, recheck the solution and wait until the WBEM test becomes successful.
   

Once successful, proceed to add the desired host in VembuBDR or VembuVMBackup. If the issue persists, consider reaching out to Vembu support for further assistance.