Youtube-dl is not working like it used to.
andy@7_~/Downloads$ youtube-dl https://www.youtube.com/watch?v=PvxQ57BvAQQ
[youtube] PvxQ57BvAQQ: Downloading webpage
[youtube] PvxQ57BvAQQ: Downloading video info webpage
ERROR: PvxQ57BvAQQ: YouTube said: Invalid parameters.Whatever version of youtube-dl is this anyway?
You can check with the command:
youtube-dl --version
Generally speaking, if it's more than three months old, youtube-dl will fail to function since YouTube changes their site a lot.
you must add -f (number code)
example:
youtube-dl -f 18 https://www.youtube.com/watch?v=PvxQ57BvAQQ
18 is for 640x360 video
for others code use -F to check it
youtube-dl -F https://www.youtube.com/watch?v=PvxQ57BvAQQ
I got invalid parameters with both your examples.
The version is 2018.03.14
youtube-dl is already the newest version (2018.03.14-1ubuntu18.04.1).
Seems strange that a 2 yr version is the newest?
try to update your youtube-dl first.
this is from youtube-dl github webpages
sudo -H pip install --upgrade youtube-dl
latest version is youtube-dl-2020.6.16.1
sudo -H pip install --upgrade youtube-dl
sudo: pip: command not found
This should fix.
sudo ln -s /usr/bin/python3 /usr/bin/python
More info
install pip
sudo apt install python3-pip
Still Invalid parameters.
Appreciate your help.
ClipGrab can download the videos using a GUI.
sorry!
not python3-pip but
sudo apt install python-pip
once again sorry, my bad 
after doing that, you can update your youtube-dl using:
sudo -H pip install --upgrade youtube-dl
Sucess. 
youtube-dl -f 18 https://www.youtube.com/watch?v=PvxQ57BvAQQ
Where does the 18 come from?
youtube-dl -F https://www.youtube.com/watch?v=PvxQ57BvAQQ
[youtube] PvxQ57BvAQQ: Downloading webpage
[info] Available formats for PvxQ57BvAQQ:
format code extension resolution note
249 webm audio only tiny 53k , opus @ 50k (48000Hz), 1.41MiB
250 webm audio only tiny 66k , opus @ 70k (48000Hz), 1.79MiB
140 m4a audio only tiny 130k , m4a_dash container, mp4a.40.2@128k (44100Hz), 3.97MiB
251 webm audio only tiny 145k , opus @160k (48000Hz), 4.01MiB
278 webm 192x144 144p 76k , webm container, vp9, 30fps, video only, 2.06MiB
160 mp4 192x144 144p 84k , avc1.4d400c, 30fps, video only, 1.39MiB
242 webm 320x240 240p 163k , vp9, 30fps, video only, 3.25MiB
133 mp4 320x240 240p 184k , avc1.4d400d, 30fps, video only, 2.44MiB
243 webm 480x360 360p 300k , vp9, 30fps, video only, 5.44MiB
134 mp4 480x360 360p 475k , avc1.4d401e, 30fps, video only, 5.70MiB
244 webm 640x480 480p 552k , vp9, 30fps, video only, 8.80MiB
135 mp4 640x480 480p 871k , avc1.4d401e, 30fps, video only, 11.08MiB
18 mp4 480x360 360p 393k , avc1.42001E, 30fps, mp4a.40.2@ 96k (44100Hz), 12.07MiB (best)