Well, I'm attempting to download a you tube video so I type:

  $ youtube-dl -o Darktable-1-Getting-Started "
https://www.youtube.com/watch?v=IwoHkXqHTCc"
[youtube] IwoHkXqHTCc: Downloading webpage
[youtube] IwoHkXqHTCc: Downloading video info webpage
[youtube] IwoHkXqHTCc: Extracting video information
[youtube] IwoHkXqHTCc: Downloading DASH manifest
[download] Destination: Darktable-1-Getting-Started.f271
[download] 100% of 69.56MiB in 00:09
[download] Destination: Darktable-1-Getting-Started.f251
[download] 100% of 5.44MiB in 00:00
[ffmpeg] Merging formats into "Darktable-1-Getting-Started.webm"
Deleting original file Darktable-1-Getting-Started.f271 (pass -k to keep)
Deleting original file Darktable-1-Getting-Started.f251 (pass -k to keep)

but when I try to view the video  this error appears:

No suitable decoder module:

VLC does not support the audio or video format "VP90". Unfortunately there
is no way for you to fix this.

No suitable decoder module:

VLC does not support the audio or video format "undf". Unfortunately there
is no way for you to fix this.


so I looked at the man page  and found --recode-video so I figured I'd try
that.


 $ youtube-dl -o Darktable-1-Getting-Started "
https://www.youtube.com/watch?v=IwoHkXqHTCc" --recode-video mp4
[youtube] IwoHkXqHTCc: Downloading webpage
[youtube] IwoHkXqHTCc: Downloading video info webpage
[youtube] IwoHkXqHTCc: Extracting video information
[youtube] IwoHkXqHTCc: Downloading DASH manifest
[download] Darktable-1-Getting-Started.webm has already been downloaded and
merged
[ffmpeg] Converting video from webm to mp4, Destination:
Darktable-1-Getting-Started.mp4
ERROR: [aac @ 0x1465460] The encoder 'aac' is experimental but experimental
codecs are not enabled, add '-strict -2' if you want to use it.

When I look in the file's folder I see two files have been created, an mp4
file and a webm file. When I open the mp4 (the icon of which shows a clock
on a checker field) file nothing happens. When I try to open the webm file
(the icon of which is a video frame) the previously mentioned error appears
and the the progress bar starts counting up.

So I think that too get help I better open the files from the terminal to
get a proper error:

$ vlc *webm
VLC media player 2.1.6 Rincewind (revision 2.1.6-0-gea01d28)
[0xf4c118] main libvlc: Running vlc with the default interface. Use 'cvlc'
to use vlc without interface.
"sni-qt/11052" WARN  10:51:50.389 void
StatusNotifierItemFactory::connectToSnw() Invalid interface to SNW_SERVICE
[0x7f4010c01968] mkv demux error: unknown codec id=`A_OPUS'
[0x7f4010c0bbe8] main decoder error: no suitable decoder module for fourcc
`VP90'. VLC probably does not support this sound or video format.
[0x7f4010c0bbe8] main decoder error: no suitable decoder module for fourcc
`undf'. VLC probably does not support this sound or video format.

$ vlc *mp4
VLC media player 2.1.6 Rincewind (revision 2.1.6-0-gea01d28)
[0x1790118] main libvlc: Running vlc with the default interface. Use 'cvlc'
to use vlc without interface.
"sni-qt/11040" WARN  10:49:31.678 void
StatusNotifierItemFactory::connectToSnw() Invalid interface to SNW_SERVICE
[0x7fe33c0012e8] main stream error: cannot pre fill buffer

-- 
:-)~MIKE~(-: