PML setup error in HFSS

[mnhasant]

Hi,
I designed vivaldi in HFSS.
then I placed two identical vivaldi with 80 cm separation.
Since the problem size is huge, I don't want to use radiation boundary box. Instead I want to use PML

But whenever I am trying to setup PML, it provides an error message attached below.

please help me. I have been through Help documents of HFSS, memorized word for word in documentations. Searched other thread related to PML, but didn't find specific solution to my problem. If I could get any help from other online sources, I would not post here. So kindly do not give me gaseous suggestion like "there are plenty of online help of pml availabe".
Thanks

 

[PlanarMetamaterials]

Why is the antenna selected in your image?

I would still use a radiation boundary on the vacuum box in this case.

 

[mnhasant]

Hi,
I also tried to use an air box and defined the faces of air box as PML but error message won't simulation to run

 

[mnhasant]

I solved my problem by myself. Simulation ran well with PML boundaries.