tkinter.TclError: couldn't connect to display “localhost:18.0”

Question

I was trying to run a simulation (written in python) in the central server, and when simulation is finished, move saved figure file / saved data file to my local PC, by conn

Answer 1

The problem is that you are using an interactive backend which is trying to create figure windows for you, which are failing because you have disconnected the x-server that was available when you started the simulations.

Change your imports to

import matplotlib
matplotlib.use('pdf')
import matplotlib.pyplot as plt

Answer 2

Generate images without having a window appear (background)

use a non-interactive backend (see What is a backend?) such as Agg (for PNGs), PDF, SVG or PS. In your figure-generating script, just call the matplotlib.use() directive before importing pylab or pyplot:

import matplotlib
matplotlib.use('Agg')
import matplotlib.pyplot as plt

plt.plot([1,2,3])
plt.savefig('myfig')

Note: This answer was in short mentioned in a comment. I put it here as an answer to increase visibility since it helped me and I was lucky enough that I decided to read the comments.