How to fix youtube-dl error : unable to open for writing: [Errno 21] Is a directory?

The -o option tells youtube-dl how to format the target path and name of your videos (not only the directory). Kind of counter-intuitive.

From the youtube-dl(1) README or manual:

-o, --output TEMPLATE

Output filename template, see the "OUTPUT TEMPLATE" for all the info

So you need something like:

youtube-dl -o "$HOME/Videos/%(title)s.%(ext)s" 'http://www.youtube.com/watch?v=P0YiWsAM0O8'

To specify something other than the default of %(title)s-%(id)s.%(ext)s. I am guessing you most likely want to just move into your ~/Videos folder and run youtube-dl with no options there though.