Can't apply transition effect between clips in Premiere Pro
When you apply a transition between two clips, Premiere needs to use the frames of footage for each clip that go to the start and end of the transition. If there's no footage past that point, it won't allow you to apply the transition. 

Consider my illustration below. 

You may need to have some footage remaining after the end of your clip to allow for this. Re-import or trim your clips to allow some footage to 'show' underneath the dip to white.