I’ve encountered the following error message in Server Manager on a Windows Server 2016 box, when trying to remotely manage another server. The only thing different from a more common situation, was that these 2 servers were not domain joined at all, and had different Administrator accounts and passwords.
In order to fix this, you just need to follow the below steps.
Step 1 – Check TrustedHosts
On the server where you want to manage remote machines from (so the client), please run the following command in a privileged PowerShell session:
Get-Item WSMan:\localhost\Client\TrustedHosts | select name,value | format-list
This command show return an empty list, or a list that does not contain the server you want to manage.
Step 2 – Add Server to TrustedHosts
On the server you want to manage remote machines from, please run the following command in a privileged PowerShell session:
Set-Item wsman:\localhost\Client\TrustedHosts "computer_name" -Concatenate -Force
If you run the command at step 1, you should now see a new addition to the list.
Step 3 – Store Remote Server Credentials
Using CMDKEY, on the server where you want to manage remote machines from, perform the following command, for each server you want to remotely manage:
cmdkey /add:computer_name /user:Administrator /pass:Password
The last step, on the managing server, using Server Manager, right click the entry that was throwing errors and click remove. Now try to add the target server one more time, and it should give you a successful result (it should say Online under Manageability)