Why is the following AC analysis of CMOS inverter resulted in zero AC gain (vout/vin) ?
Removing Cout solves the zero AC gain issue. Remember the RC constant = 100G * 1n = 100s, so 0.01Hz filtering on the AC gain plot result.
Your source is DC coupled, with a very low source impedance, and it's pulling the input down out of the active region.
You need to put a capacitor in series with RS1, so that Rf can then bias the inverter back into the active region.
As others have said Rf is probably a bit big, normal values are 1E6 - 1E7.