A component that has already been defined (for example, a plate or a stiffener) does not appear in the selection box or is grayed out if it is located below the referencing component in the component list or is deactivated.
In the Steel Joints add-on, all components are processed strictly sequentially from top to bottom.
A component can therefore only access structural components that have already been defined in the list (that is, in a line above it).
If the desired reference is located below in the structure, it does not yet exist from a processing point of view—even if it is geometrically visible.
Since the order of the components is crucial, it is necessary to create the referenced component before the referencing component. If this is not the case, you need to delete the component in question and recreate it in the correct order.