Structures are a special case of scalar parameters. In case of structures, an individual variable doesn’t have to be assigned to the whole structure, but rather, to each element of the structure.
Please use the Edit link to open up an additional dialogue for assigning structure elements as shown in the screenshot. Analogous to purely scalar parameters applies also for structural elements: Imports may contain fixed values or variables. Of course the value fields in all cases can also be left blank, if the relevant parameter is not required.