Illegal usage of S-Matrix, No impedance found error in cst

Dear all I am designing a split ring resonator which is exited by waveguide port but when i simulate the design I get two error message "Illegal usage of S-Matrix, No impedance found error in cst and the other one is "Time Domain Solver (HEX) terminated abnormally" does anyone have any idea what these errors are and how I can solve them.
Regards