Problem with current flow line of Lumped RLC boundary in HFSS

Hi all,
I want to assign boundary of this box (marked with blue line) using Lumped RLC. But I cannot assign the new current flow line on this box; since I want express it as a R/L/C parameters. It shows that "the new assign current flow line can be created on the selected surface". How can I solve this problem.

I have attached the problem with jpg file.

Welcome anas171,

The object should be a surface, not a volumetric structure. You can select one face of the box to accomplish this.