OK, I missed the port impedances. You set Port1 to 600 Ohm, Port2 to 50 Ohm.
You cannot do that. Spectre n-port device accepts-IMHO- Touchstone or similar s-parameters definition format and there is only ONE Port Impedance in it.
So you should enter either 50 Ohm OR 600 Ohm but not both. If you check Touchstone file by notepad, you will see that the Common Port impedance.
ADS creates mixed mode touchstone file.
View attachment 190427
You have to search maybe a different format than Touchstone 1.0. There might be some options to convert this onto 2.0