This code is meant to take a filename and a list of integers as arguments.  The filename is required, the integers are optional:

import argparse
parser = argparse.ArgumentParser()
parser.add_argument('filename')
parser.add_argument('-L', metavar='integer', type=int, nargs='+')
args = parser.parse_args()
print(args)  # see what we got

It produces the following help message:
usage: demo.py [-h] [-L integer [integer ...]] filename

However, the filename argument does not work if it's given in that position (after the list of ints).  Instead, it tries to use filename as another list element:

$ python demo.py -L 1 2 3 test.txt
usage: demo.py [-h] [-L integer [integer ...]] filename
demo.py: error: argument -L: invalid int value: 'test.txt'

Changing the order of the arguments works as intended:

$ python demo.py test.txt -L 1 2 3 
Namespace(L=[1, 2, 3], filename='test.txt')

Probably the simplest fix would be to amend the help message to show the positional argument before the list:

usage: demo.py [-h] filename [-L integer [integer ...]]