Premiere: Add transition to trimmed clip In the screenshot below, I trimmed the left clip (by dragging its right edge to the left) in order to remove a couple of seconds from the end.

But the applied morph cut transition is only added to the right clip when I drag it there (which doesn't work, just shows an error in the video). How can I fix this?
Delete the transition, click on the edge between the two clips, so it goes red like this:

Then apply the transition.
If you cannot, it's because one of the clips does not have any more frames to the other side of it with which to complete the transition.  ie if your transition is 10 frames long, you need five extra frames of footage on the left and right edges for it to work.