Sort list of files based on creation date in Python

I've looked around and can't find a specific answer to my solution and my brain is fried at this point. I'm trying to create an mp4 video based on some .bmp files in a folder. However, I want the files ordered by earliest modified date for the video. So I'm using the oldest date modified. I found some stuff on here about using os.path.getmtime, however if I add that it's telling me it can't find the file. I'm guessing it's because the files are located on a network and not in my local path where python is installed. Here is my code. I've confirmed everything else works, so all I need is to find out how to sort the files.

import cv2
import numpy as np
import os
from os.path import isfile, join

#change this to the path where the pictures are located
pathIn= #MyPathWhichIsOnANetworkNotWherePythonIsInstalled

#input your video name & video type:
vid_name = "FirstCalPics.mp4"

#change this to the path where the video should be saved:
pathSave = #AlsoAPathWhichIsOnANetworkNotWherePythonIsInstalled

#set your fps here:
fps = 10

pathOut = pathSave + vid_name

fourcc = cv2.VideoWriter_fourcc(*'mp4v')

frame_array = []
files = [f for f in os.listdir(pathIn) if isfile(join(pathIn, f))]

#Sort files based on date modified:
files.sort(key=os.path.getmtime)   #<--- HERE'S THE ISSUE

for i in range(len(files)):
    filename=pathIn + "\\" + files[i]
    #reading each files
    img = cv2.imread(filename)
    height, width, layers = img.shape
    size = (width,height)
    
    #inserting the frames into an image array
    frame_array.append(img)
out = cv2.VideoWriter(pathOut, fourcc, fps, size)

for i in range(len(frame_array)):
    # writing to a image array
    out.write(frame_array[i])
out.release()