PML setup error in HFSS

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

Why is the antenna selected in your image?

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

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

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