The error message Failed to access target system. Please check credentials and firewall settings on the target system to ensure accessibility: Access is denied. (Exception from HRESULT: 0x80070005 (E_ACCESSDENIED)) can be caused by 2 primary issues. I will discuss the first 2 troubleshooting steps I always work through before attempting further troubleshooting steps.
Step 1 (Validate Credential using RDP):
RDP from the MID server to the Remote Windows device to validate the Windows credential as you have done. This is excellent way to validate the credential. BUT this step only validates the credential - because RDP communicates on port 3389 to login to the Remote Windows device while ServiceNow Discovery uses a different range of ports for login and discovery communication.
Step 2 (Validate WMI port 135 and port ranges 49152 - 65535 are open):
Validate the ServiceNow Discovery WMI ports between the MID Server and the Remote Windows device are open in the Firewall. ServiceNow Discovery uses WMI for discovery, therefore port 135 from the MID Server to the Remote Windows device must be open for initial communication AND high ports 49152 - 65535 must be open for the remainder of the communication. Even though this is a large range of open ports, only a portion of this range are dynamically allocated.
If you are in a lab environment, a quick way to validate step 2 is to turn off the Remote Windows or SEP firewall and initiate discovery. In a production environment, I review the firewall logs looking for blocks.
A slightly more helpful error message might look like: Failed to access target system. Please check credentials (Step 1: Validate Credential using RDP from the MID Server) and firewall settings (Step 2: Validate WMI port 135 and high ports 49152 - 65535 are open) on the target system to ensure accessibility: Access is denied. (Exception from HRESULT: 0x80070005 (E_ACCESSDENIED)).
