How to get frequency response from LTSpice?
It appears that the NMOS / PMOS models you have chosen do not have any frequency-dependent effects. The rest of your circuit also has no capacitors or inductors. As a result, the gain of the circuit is independent of frequency. Therefore, the plotted frequency response is a flat horizontal line.
From your schematic screenshot, there is a line cut off at the top that says (kp=... vto=... lambda=...). All three of these parameters (kp, vto, and lambda) specify only the DC behavior of a MOSFET.
You'll need to add other parameters to specify frequency-dependent effects.
See LTspice MOSFET model help to learn more about these parameters. Alternatively, you can use a pre-made MOSFET model that already includes these parameters.