FileNotFoundError after upscaling

[Gulli03]

(vus) G:\Video-upscaling>python Videoupscale.py
100.00%  elpsed: 97.05s  remaining:  0.00s
Traceback (most recent call last):
  File "Videoupscale.py", line 5, in <module>
    pyanime4k.upscale_videos(pathlib.Path('source.mp4'))
  File "C:\Users\gladi\AppData\Local\Programs\Python\Python38\lib\site-packages\pyanime4k\upscale.py", line 278, in upscale_videos
    ffmpeg_handler.migrate_audio_streams(
  File "C:\Users\gladi\AppData\Local\Programs\Python\Python38\lib\site-packages\pyanime4k\ffmpeg_handler.py", line 31, in migrate_audio_streams
    output.run()
  File "C:\Users\gladi\AppData\Local\Programs\Python\Python38\lib\site-packages\ffmpeg\_run.py", line 313, in run
    process = run_async(
  File "C:\Users\gladi\AppData\Local\Programs\Python\Python38\lib\site-packages\ffmpeg\_run.py", line 284, in run_async
    return subprocess.Popen(
  File "C:\Users\gladi\AppData\Local\Programs\Python\Python38\lib\subprocess.py", line 854, in __init__
    self._execute_child(args, executable, preexec_fn, close_fds,
  File "C:\Users\gladi\AppData\Local\Programs\Python\Python38\lib\subprocess.py", line 1307, in _execute_child
    hp, ht, pid, tid = _winapi.CreateProcess(executable, args,
FileNotFoundError: [WinError 2] Das System kann die angegebene Datei nicht finden

After it has finished with the upscaling it gives me this error.
My code is as follows:

import pathlib
import pyanime4k

pyanime4k.upscale_videos(pathlib.Path('source.mp4'))