CST - Query regarding finding the power loss density in CST

Dear all I want to find the Power loss density among the walls of a cavity in CST, I have modeled it using Vacuum and background as PEC. As I tried to simulate using Eigen mode solver, Frequency domain solver, and Time domain solver also to find the power loss density in the cavity but iam getting a warning as mentioned in the screenshot. 

So in this regard i need help regarding which solver to be used and how to find it. Thanks in advance

CST Studio Suite