This sidequest will unlock after you finish the quest chain with The Warchief’s Trail and Revenge of the Nora in it. Speak to Sona and she’ll request that you do a few minor task for her. One of them, the Chillwater, should be instantly completed if you’ve been looting everything you’ve killed. If not, the quest will give you a marker where to find some machines to kill in order to get Chillwater. The others are simple enough. Follow your waypoints first to the watchtower and deliver the arrows, then head for the Lancehorn site to get a Lancehorn lens. These are quick machines, so your best bet is to stealth kill them to get the part you need.
When you’ve completed the three tasks, head back to Sona at Mother’s Crown and speak to her to complete the Errand.