Failed to download from youtube

[gwal1]

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• Put an x into all the boxes [ ] relevant to your issue (like this: [x])
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Make sure you are using the latest version: run youtube-dl --version and ensure your version is 2018.11.23. If it's not, read this FAQ entry and update. Issues with outdated version will be rejected.

• I've verified and I assure that I'm running youtube-dl 2018.11.23

Before submitting an issue make sure you have:

• At least skimmed through the README, most notably the FAQ and BUGS sections
• Searched the bugtracker for similar issues including closed ones
• Checked that provided video/audio/playlist URLs (if any) are alive and playable in a browser

What is the purpose of your issue?

• Bug report (encountered problems with youtube-dl)
• Site support request (request for adding support for a new site)
• Feature request (request for a new functionality)
• Question
• Other

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The command line typed is:
youtube-dl -v https://www.youtube.com/watch?v=z_aVNv_gNdM

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[debug] System config: []
[debug] User config: []
[debug] Custom config: []
[debug] Command-line args: [u'-v', u'https://www.youtube.com/watch?v=z_aVNv_gNdM']
[debug] Encodings: locale UTF-8, fs UTF-8, out UTF-8, pref UTF-8
[debug] youtube-dl version 2018.05.18
[debug] Python version 2.7.13 (CPython) - Linux-4.9.0-8-amd64-x86_64-with-debian-9.6
[debug] exe versions: ffmpeg 3.2.12-1, ffprobe 3.2.12-1
[debug] Proxy map: {}
[youtube] z_aVNv_gNdM: Downloading webpage
[youtube] z_aVNv_gNdM: Downloading video info webpage
[youtube] z_aVNv_gNdM: Extracting video information
[youtube] {18} signature length 46.40, html5 player vflBGiA6J
[youtube] z_aVNv_gNdM: Downloading player https://www.youtube.com/yts/jsbin/player-vflBGiA6J/en_US/base.js
ERROR: Signature extraction failed: Traceback (most recent call last):
File "/home/stefano/.local/lib/python2.7/site-packages/youtube_dl/extractor/youtube.py", line 1192, in _decrypt_signature
video_id, player_url, s
File "/home/stefano/.local/lib/python2.7/site-packages/youtube_dl/extractor/youtube.py", line 1103, in _extract_signature_function
res = self._parse_sig_js(code)
File "/home/stefano/.local/lib/python2.7/site-packages/youtube_dl/extractor/youtube.py", line 1164, in _parse_sig_js
jscode, 'Initial JS player signature function name', group='sig')
File "/home/stefano/.local/lib/python2.7/site-packages/youtube_dl/extractor/common.py", line 883, in _search_regex
raise RegexNotFoundError('Unable to extract %s' % _name)
RegexNotFoundError: Unable to extract Initial JS player signature function name; please report this issue on https://yt-dl.org/bug . Make sure you are using the latest version; see https://yt-dl.org/update on how to update. Be sure to call youtube-dl with the --verbose flag and include its complete output.
(caused by RegexNotFoundError(u'Unable to extract \x1b[0;34mInitial JS player signature function name\x1b[0m; please report this issue on https://yt-dl.org/bug . Make sure you are using the latest version; see https://yt-dl.org/update on how to update. Be sure to call youtube-dl with the --verbose flag and include its complete output.',)); please report this issue on https://yt-dl.org/bug . Make sure you are using the latest version; see https://yt-dl.org/update on how to update. Be sure to call youtube-dl with the --verbose flag and include its complete output.
Traceback (most recent call last):
File "/home/stefano/.local/lib/python2.7/site-packages/youtube_dl/extractor/youtube.py", line 1192, in _decrypt_signature
video_id, player_url, s
File "/home/stefano/.local/lib/python2.7/site-packages/youtube_dl/extractor/youtube.py", line 1103, in _extract_signature_function
res = self._parse_sig_js(code)
File "/home/stefano/.local/lib/python2.7/site-packages/youtube_dl/extractor/youtube.py", line 1164, in _parse_sig_js
jscode, 'Initial JS player signature function name', group='sig')
File "/home/stefano/.local/lib/python2.7/site-packages/youtube_dl/extractor/common.py", line 883, in _search_regex
raise RegexNotFoundError('Unable to extract %s' % _name)
RegexNotFoundError: Unable to extract Initial JS player signature function name; please report this issue on https://yt-dl.org/bug . Make sure you are using the latest version; see https://yt-dl.org/update on how to update. Be sure to call youtube-dl with the --verbose flag and include its complete output.
Traceback (most recent call last):
File "/home/stefano/.local/lib/python2.7/site-packages/youtube_dl/YoutubeDL.py", line 792, in extract_info
ie_result = ie.extract(url)
File "/home/stefano/.local/lib/python2.7/site-packages/youtube_dl/extractor/common.py", line 503, in extract
ie_result = self._real_extract(url)
File "/home/stefano/.local/lib/python2.7/site-packages/youtube_dl/extractor/youtube.py", line 1907, in _real_extract
encrypted_sig, video_id, player_url, age_gate)
File "/home/stefano/.local/lib/python2.7/site-packages/youtube_dl/extractor/youtube.py", line 1202, in _decrypt_signature
'Signature extraction failed: ' + tb, cause=e)
ExtractorError: Signature extraction failed: Traceback (most recent call last):
File "/home/stefano/.local/lib/python2.7/site-packages/youtube_dl/extractor/youtube.py", line 1192, in _decrypt_signature
video_id, player_url, s
File "/home/stefano/.local/lib/python2.7/site-packages/youtube_dl/extractor/youtube.py", line 1103, in _extract_signature_function
res = self._parse_sig_js(code)
File "/home/stefano/.local/lib/python2.7/site-packages/youtube_dl/extractor/youtube.py", line 1164, in _parse_sig_js
jscode, 'Initial JS player signature function name', group='sig')
File "/home/stefano/.local/lib/python2.7/site-packages/youtube_dl/extractor/common.py", line 883, in _search_regex
raise RegexNotFoundError('Unable to extract %s' % _name)
RegexNotFoundError: Unable to extract Initial JS player signature function name; please report this issue on https://yt-dl.org/bug . Make sure you are using the latest version; see https://yt-dl.org/update on how to update. Be sure to call youtube-dl with the --verbose flag and include its complete output.
(caused by RegexNotFoundError(u'Unable to extract \x1b[0;34mInitial JS player signature function name\x1b[0m; please report this issue on https://yt-dl.org/bug . Make sure you are using the latest version; see https://yt-dl.org/update on how to update. Be sure to call youtube-dl with the --verbose flag and include its complete output.',)); please report this issue on https://yt-dl.org/bug . Make sure you are using the latest version; see https://yt-dl.org/update on how to update. Be sure to call youtube-dl with the --verbose flag and include its complete output.

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I cannot download any file format of the chosen audio/video from youtube.

Thanks for support