If you use Python scripts or apps that depend on python to run, you may see an error message similar to the following on Debian Bullseye (possibly other distributions or versions as well). This occurs because the python application has been replaced by python3 in Debian Bullseye and so the system cannot find anything simply called python.
Fortunately the fix is very simple and that is to place a symlink to python3 somewhere in the application path and call it python. For example in my system, I found the python3 app was installed in /usr/bin, so I navigated to this directory in a root shell and then entered the command:
ln -s python3 python
After this, youtube-dl works again.