I learn about ANSYS capabilities by participation on this forum as well as share what I know. I haven't used Substructuring, but I am curious about how to do so.
First I would ask which license you are using: Student (limited) or Research (unlimited) and which version 18.2, 19.1 etc. I have access to both a Commercial (unlimited) license on one computer and a Student license on a second computer. That way I can see when a problem exceeds the Student license limits. I suspect that Substructuring is not supported on the Student license.
Second, I would direct you to the ANSYS Help file in this area:
I haven't read the pages and pages that are in the guide, I will do that later. I wanted to jump in with the info in Sandeep's link.
I put a Command object in a model like this:
I got a lot of errors, so I read some of the ANSYS Help on this topic and learned that Substurturing (at least in 18.2) requires Linear Elements. So I went to the Mesh Details and changed the Element Order from Program Controlled to Linear. Then I hit Solve again and it seems like the solver started but is waiting for something and hasn't finished. I may have to go read the ANSYS Help again!