if you see this page it means that ffmpeg is installed
but you still need to add it as a System Variable so it can work
if you know how it works good for you but for everyone else here a guide
Press Windows + R
now a small window appeared
you need to put this into the entry box
rundll32.exe sysdm.cpl,EditEnvironmentVariables
like this
then you press the ok button after that a window should appear like this one
then you double click on "Path" on the first window
after that you press on New then a small textbox should appear in that textbox you are going to type in C:\ffmpeg\bin
and now you are done! don't forget to restart yt2file