Can't play and covert with Prism

[Meylh]

I can't convert my videos and I can't even play them. Every time I click "Convert", a message comes in that says: Download install-on-demand ffmpeg4. I have no idea what that means. Then after like 5 seconds another message comes from the Install Component that says: Can't get <<class cfol>> "Application Support" of <<class cfol>> "Library" of item "Meyling" of Application "Finder". What am I supposed to do? Any help is appreciated.