Common-Emitter NPN Transistor Amplifier Simulation Failed
V1 0 0 DC 20


The net connections for V1 are 0 and 0 i.e. you have both sides of the 20 volt battery connected to the same net. If you looked at your input waveform as it settles down you can see there is no bias voltage on the square wave and this also implies the supply voltage is connected incorrectly.

Compare this with R4: -

R4 0 in  10K


Clearly that is correctly connected between node 0 and node "in".

Aha, here's the problem: -



You have the 20 volt supply shorted out!