You haven't provided the full path to ffmpeg
See:
action:Execute a Shell Script [Keyboard Maestro Wiki]
- The
$PATH
environment variable you may have set in Terminal is not used.
(The search paths defined in the Terminal.app shell instance are not defined in the vanilla shell instance used by Execute Shell Script
)
In Terminal.app, you can, of course find the full path with an incantation like:
which ffmpeg
and the full path may be something like
/usr/local/bin/ffmpeg