The source which you have used is wrong.It's a AC source and cannot be used in s-parameter simulations.
You have to use "psin" or "port" element instead of it.These components have internal source impedance ( e.g. 50 Ohm by default) and you should define this component in s-parameter simulation setup ( if you do 1-Port s-parameter simulation, you should add this in port definition).
In additional to, you may use "psin" or "port" element for other ports and if you add these elements in s-parameter simulation setup, you can do 4-Ports s-parameter simulation without adding extra resistors.
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