PermissionError while running function

[Laplace12]

Well, it's not a file but a directory, but as I understand that shouldn't make a difference? The directory does exist, and the path should be correct as it works when calculating the files in that directory in the same function, and I use the same mca_file there. The complete function is this

def function(n_pts, n_chnl, mca_file):
    print('Checking that n_files divided by n_pts is an integer...')
    n_files = 0
    for path in pathlib.Path(mca_file).iterdir():
        if path.is_file():
           n_files += 1
    if n_files % n_pts == 0:
        print('Done!')
    else:
        print('Not an integer, check the number of files in your mca folder')
        raise SystemError
    print('Summing up mcas in mca_file...')
    mcadata = np.zeros([n_chnl, n_pts])

    for energy_point in range(n_pts):
        current_mca = os.listdir(mca_file)

        for mca in current_mca:
            filename = mca_file.format(mca)
            print('DEBUG: is file? ', os.path.isfile(filename), filename)
            mcadata[:, energy_point] += np.loadtxt(filename)
            #mcadata[:, energy_point] += np.loadtxt(mca_file.format(mca))
    print('Done!')
    return mcadata

and the code does the first part well, dividing n_files with n_pts using the same exact directory named as 'mca_file'. Perhaps the os.listdir command doesn't work here, then? Also, huge thanks for your help so far!