HFSS floquet port question

Hi I am a new user of Ansys HFSS. I am currently having some floquet port difficulties for my design. I have done the tutorial for floquet ports that is available in the learning guide. However, when I try to assign the floquet port at one of the faces this error message shows up: "The floquet port excitation must touch the background". Anyone have any idea why this is happening?

I believe the floquet port must be on the outside perimiter of your setup, i.e. touching the background. For example, if you have a vacuum box around your simulation object, the floquet ports must be on the surface of this box.