Hi Grant, Your clip Seg_12.mp4 has no more media on the left side of the cut. The little triangles indicate the end of a clip.
Transitions in between two clips need to have additional media beyond the cut.
Here is more information on this subject: https://www.filmimpact.com/premiere-pro-insufficient-media-this-transition-will-contain-repeated-frames/
Why do video transitions need additional media?
It may seem like everything was fine just before you applied a video transition. Why do video transitions need additional media? Take a look at this sequence with two clips sitting next to each other:

If you apply a video transition in between these clips, you can see Clip B is introduced earlier than the actual cut. Clip A continues to be visible until the end of the transition. Depending on the length of the video transition, you need media before and beyond the cut.

Solution: Trim away some of the video before adding the cross dissolve transition.
Can you follow me here?
Best regards,
Jaap