Please give the error statement along with the type.
Usually the ErrType(3) is: pin connected to some other pins but no pin to drive it.
Then, you need to add PWR_FLAG to indicate that these are power ports. For any power ports used, you need to add PWR_FLAG.
All the best...
Yes, for any power source (Vcc, Gnd) you need to add PWR_FLAG to indicate these are power ports.
Since you are using Op-amp (lm741). Try to add component form eSim_Subckt devices.
In eSim_Subckt devices, it is referred as ua741.
* Remember, once you convert kicad-ngspice, don't forget to add the path of subcircuitused.
For output plot: you can choose component fromeSIm_Plot, based on what kind of output you need to plot (log, v, i, phase).
Click on it. You will get add button.
Click on it and explore the available sub-circuits.