Garageband 11 - cannot edit music?

Hi all,

I have just upgraded to Garageband 11 and cannot work out how I can edit a music track from itunes. I normally hightlight part of the track that I want to edit but now a message appears saying analysing audio for flex timing. When I hit delete the whole track disappears.

Would be grateful of any help, step by step if possible as need to do a track urgently for a school production.

Many thanks