Troubleshooting Timeline 13 for Scroll Line Effect and Gradient Control

I’m trying to use the new timelines but having trouble with Timeline 13. I’ve followed the instructions to give the previous section div a BG color, and a z-2 class. The line that is supposed to follow the scroll isn’t appearing. If I apply a color it just becomes a solid line without any scroll effect. Am I missing something? My goal is to 1) see the actual line progress effect, and 2) be able to control its coloration (gradient as in the oft-duplicated Relume clone)