I don't understand this answer. I want my transition longer, iMovie says there's not "the clips do not have enough media" and you're recommending to shorten the clips next to the transition??? It doesn't make sense, nor does it work!

The message means that there is not enough “residual” media. Create more by trimming back a few frames of the video clips on each side of the transition. Residual media is the invisible media left over when you trim back a video clip. It is what the transition overlaps and hooks on to. So, by trimming you are adding more residual media. Trim back as much as necessary.

— Rich

You will have 2 clips where your transition will go. Cut about a second off the ends where they "touch" and you will have no problems making a transition up to 2 seconds long.

This will have to be done wherever you have a transition.

When professionals edit, they rarely use full clips because usually the beginnings and ends of clips are not the best bits, so all their clips have been trimmed fore and aft before they try to insert transitions.

Regarding professional editors and transitions . . . they almost never use them.

Watch any top professional Hollywood etc. film and you will be very hard pressed to find any transition other than straight cuts.

It's only amateurs who seem to use (or overuse) transitions.