And when I commented the voltage sources the ngspice was showing no error, how to perform post layout simulations, how to give voltage values to the spice netlist

Normally, your testbench simulation netlist would be a wrapper around the extracted netlist and include it with a

.include

statement. Then you need to instantiate the subcircuit with a call

X

of the subcircuit. A netlist that has a subcircuit is not simulatable. That subcircuit must be instantiated.