The parameter 'primary_qtree" is of type Reference Object indicated by a "R" in front of it. So now you can't use $qtree_name as a direct user-input parameter, you'll need to use a Filter.
Click on the small box which appears when you take the mouse to enter the qtree_name field. See the image below. There is a tooltip saying "Click button to search for Qtree". Click it.
Now select your filter. The primary Volume name will taken from primary_qtree.volume.name
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