The Trail Material UI is missing.

1) Create new project
2) Right click hierarchy view, create new Effects-> Particle System
2b) If Resimulate is checked in the Scene View Overlay, uncheck it, delete the system, and go back to Step 2.
3) Enable the Trails module
4) Notice that the Trails Inspector tells you to set the Trails material in the Renderer module Inspector UI
5) Unroll the Renderer module Inspector. Notice that there is no slot for the Trails Material

Workaround: with the system selected, press "Restart" in the Scene View Overlay. When the system begins to play, the UI for the Trail Material appears.

Raised here: https://forum.unity.com/threads/cannot-assign-trailmaterial-in-rederer-module-of-particlesystem.985199/