Why is there black line on the 360VR?

The video you took is beautiful. The Line appears because the video was scaled to a larger resolution and the absence of information at the edges renders as black. Top and Bottom strips (in flat) formed the black circles that appear at the poles (of 360) and the tiny gap on the right and left of the frame formed the black line. I think you used sequences of the same frame size as the GoPro stitched outputs. If not you have to make sure you do a right click > "Scale to Frame Size" on the video clips added.

If that wasn't the case you have to lessen the frame size of the sequence since edge data loss is high when scaling from 5K to 8K. The problem could have happened when you exported your video through a preset that does not match the sequence settings. You can try 5.7K which works on YouTube; I say you custom export you video in that size (5760x2880) in H.265 encoding.