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Problem configuring a visual angle

Hey there ! I’m beginning to learn to code, and I just want a stimulus to be presented on a screen at a certain degree of visual angle. Here is my code for the moment:

import numpy as np
import time
#Functions
def make_dot(number_dots):
   r = [];
   for i in range (number_dots):
       random_placex = np.random.normal()
       random_placey = np.random.normal()
       r.append(vis.Circle (win,radius = 0.08,units='deg',fillColor='white',pos=[random_placex,random_placey]))
   return r

##how many dots?
nb_dots = 50

#Make a window
win = vis.Window(size=(500,600))
dots = make_dot(nb_dots)
for i in range(len(dots)):
   dots[i].draw()
win.flip()
time.sleep(5) 

And I get the following error message:
raise ValueError(msg % monitor.name)
I guess I need to define the monitor I’m working on; But I have currently absolutely no idea how to do this, and I’m just desparate to find a solution. Could someone help me please?