When I optimize my clips on iMovie, it tells me there is an error.

On iMoive, I'm needing to make a movie for school, I've made a video on photobooth and I want to edit it on iMovie. I put the movie in my project and I go to make it fast forward, when I do, it tells me 'This clip must be optimized before its speed can be adjusted.'

I go to 'file' and go 'optimize' it then tells me 'This clip cannot be converted. Error: -50'

I really need to get this done asap 🙂 Thanks