Firstly, make sure that the remote registry is running under services.msc in all workstations. Then, check if Port 139 (File Sharing) is open on the target workstations; you should be able to telnet the targeted workstation from the probe machine. Now you'll be able to run the domain scan.
If you are unable to to connect to Admin$ share, check if you can connect to the admin$ share of the target system from the Probe machine by typing net use \\IP address of the workstation>\admin$ /user:\Username in either Run or Search. If you are not able to, then make sure Port 139 (File Sharing) is open on the target workstations.
Also, if all these conditions are met, and you still can't find any error messages, please send us the logs of the probe so that we can further analyze this issue. You will be able to find the log under this directory "C:\ManageEngine\SDPOD\Probelogs".
Click on the link below to get detailed information on scan related FAQ's,