Raghav Gururajan wrote:
>> guix environment --pure --ad-hoc youtube-viewer mpv --
>> youtube-viewer
>
> I did this and I was led to "hidden@address ~ [env]$". Anyway, I 
> never used command-line for this app.

Please run the entire command (also the part after --) on one line 
(see my follow-up mail).

> I am using the GUI. Yes, GUI is installed by default. In my 
> GNOME Dash, it appears as "GTK Youtube Viewer" with a "blue 
> camel" in the icon.

Oh.  I don't use a desktop menu.  Today I learnt that 
‘gtk-youtube-viewer’ exists :-)

Anyway, both the GTK and CLI versions invoke MPV just fine here. 
Could you try running both in a pure environment?

Kind regards,

T G-R