how to set cascading field mandatoty

[Nic Brough -Adaptavist-]

You can't do it with Field Configuration - if you do, it only makes the first option mandatory, and lets the user leave the cascaded part empty.

You'll need a validator that can check the second cascaded part of the field.  I don't know if the JSU does that.

[Elifcan Cakmak]

in order to make a custom field mandatory you need to make it required in the field configuration scheme of that project. 

or you can also use the fields required validator of JIRA suite utilites plugin. you use this validator on the transition that you want to make the field required.