This guide will show you how to get navigation working in Half-Life: Alyx.
Making AI Nav Work with Bootleg Tools
Add the following line to your FGD for HL:A if it is absent:
- @include "markup_volumes.fgd"
In your gameinfo.gi, in the Hammer section, set NavMarkupEntity to func_nav_markup (replace func_hlvr_nav_markup with func_nav_markup)
In Hammer, go to the Navigation tab and select "Create Walkable Seed", draw out a small block, and hit Enter.
Make sure NPCs are in your map, hit F9, and build the map!
The end result should look something like this: