What does "Insufficient media. This transition will contain repeated frames" mean?

Some of the responses above have suggested that you visualize A and B tracks.  However, Premiere has a facility to show you this live time.

• Click on the transition effect that is giving the problem

• Open the effects dialog and expand the timeline in the dialog.   Here you will the transition to timeline scale and the incoming and outgoing clips arranged as A and B with the transition separating

• To have a good transition, the transition has to be "covered" by both the incoming and outgoing clips.
• If that's not the case, you will see part of the transition cross hatched where it is not covered by the incoming or outgoing clips.  The cross hatch indicates the missing head and tail.
• You can use the various timeline trim tools in the effects dialog to move the transition to eliminate the cross hatching.  Note that that the effect dialog an the main timeline are synced

‌A simple way to visualize it is put the clips on two tracks, one above the other, in the exact position where you want the cut to be.

Extend each clip towards the other.  If you can't go half the length of the transition you desire you can't do a transition- you need overlap. The little triangle in the upper corner of the clip is also a warning that there is no more media to work with, that's why it's there.

From what you have said it sounds like you don't have enough "heads" and "tails" on your clips. Take off 16 frames from the end of clip A and Take off 16 frames of the beginning of clip B. This should fix your problem. I say 16 on each just to give a little extra to work with.