Premiere Pro not letting me use Constant Gain between two audio clips

You can tell by that little white triangle in the top right corner of your first clip that you've reached the end of the media. It can't do a fade/transition because there is nothing there for it to overlap with.

As Gerikp said, trim a few frame of the end of the first clip, and then try again.

Maybe your clip doesn't have enough frames in your clip handle to accommodate the transition. As a test trim off a couple of seconds from the beginning of your second clip, butt the two clips together and apply your transition again. You need enough overlapping frames in your clip for the transition to work.