https://www.pc-koubou.jp/magazine/4238
マルコフ解析と形態素解析を使い適当なテキストファイルを与えたらそれを材料に自動で文章を生成するものを作りたく、このリンクを参考にし、python3.7.2、mecab-0.996.exeをインストールした後コマンドプロンプトで
learn.pyを実行したところこのようなエラーが出ました。

C:\Users\Desktop>python learn.py

Traceback (most recent call last):
  File "learn.py", line 98, in <module>
    main()
  File "learn.py", line 84, in main
    print(''.join(sentence.split()))    # need to concatenate space-splitted text
AttributeError: 'NoneType' object has no attribute 'split'

なおlearn.pyは以下の通りです。

#!/usr/bin/env python3
# -*- coding:utf-8 -*-
from glob import iglob
import re

import MeCab
import markovify


def load_from_file(files_pattern):
    """read and merge files which matches given file pattern, prepare for parsing and return it.
    """

    # read text
    text = ""
    for path in iglob(files_pattern):
        with open(path, 'r') as f:
            text += f.read().strip()

    # delete some symbols
    unwanted_chars = ['\r', '\u3000', '-', '｜']
    for uc in unwanted_chars:
        text = text.replace(uc, '')

    # delete aozora bunko notations
    unwanted_patterns = [re.compile(r'《.*》'), re.compile(r'［＃.*］')]
    for up in unwanted_patterns:
        text = re.sub(up, '', text)

    return text


def split_for_markovify(text):
    """split text to sentences by newline, and split sentence to words by space.
    """
    # separate words using mecab
    mecab = MeCab.Tagger()
    splitted_text = ""

    # these chars might break markovify
    # https://github.com/jsvine/markovify/issues/84
    breaking_chars = [
        '(',
        ')',
        '[',
        ']',
        '"',
        "'",
    ]

    # split whole text to sentences by newline, and split sentence to words by space.
    for line in text.split():
        mp = mecab.parseToNode(line)
        while mp:
            try:
                if mp.surface not in breaking_chars:
                    splitted_text += mp.surface    # skip if node is markovify breaking char
                if mp.surface != '。' and mp.surface != '、':
                    splitted_text += ' '    # split words by space
                if mp.surface == '。':
                    splitted_text += '\n'    # reresent sentence by newline
            except UnicodeDecodeError as e:
                # sometimes error occurs
                print(line)
            finally:
                mp = mp.next

    return splitted_text


def main():
    # load text
    rampo_text = load_from_file('hoge.txt')

    # split text to learnable form
    splitted_text = split_for_markovify(rampo_text)

    # learn model from text.
    text_model = markovify.NewlineText(splitted_text, state_size=5)

    # ... and generate from model.
    sentence = text_model.make_sentence()
    print(splitted_text)
    print(''.join(sentence.split()))    # need to concatenate space-splitted text

    # save learned data
    with open('learned_data.json', 'w') as f:
        f.write(text_model.to_json())

    # later, if you want to reuse learned data...
    """
    with open('learned_data.json') as f:
        text_model = markovify.NewlineText.from_json(f.read())
    """


if __name__ == '__main__':
    main()

また使用したテキストファイルは、メモ帳で適当な文章を書きhoge.txtとデスクトップに保存したやつです。
以上のようなエラーを解決するために、learn.pyのどれを書き直す/書き足せばよいでしょうか。なお上に貼ったリンクではpython3 learn.pyと実行するのが良いと最後に書いてあるのですがそれだと自分のは

C:\Users\Desktop>python3 learn.py
'python3' は、内部コマンドまたは外部コマンド、
操作可能なプログラムまたはバッチ ファイルとして認識されていません。

と表示されます。