I want to monitor weblogic server but when I add my weblogic server into  Applications Manager 12 then I got a Failed message

This is check:

This is result after I clicked to add:

The Diagnose the problem said that:  Host Name is empty 

But I think I passed when I test so Hostname is fine, right?

* More question:

I have 2 server weblogic:

10.0.119.21

10.0.119.22

And the Admin node I run on 1 server (10.0.119.21) 

=> I have to add both 2 servers or only need to add an Admin node ?

Thank you very much